diff --git a/src/wp-includes/class-wp-image-editor.php b/src/wp-includes/class-wp-image-editor.php
index 11210180b2..0bc7ee6aee 100644
--- a/src/wp-includes/class-wp-image-editor.php
+++ b/src/wp-includes/class-wp-image-editor.php
@@ -591,13 +591,11 @@ abstract class WP_Image_Editor {
 	 * @return string|false
 	 */
 	protected static function get_extension( $mime_type = null ) {
-		$extensions = explode( '|', array_search( $mime_type, wp_get_mime_types(), true ) );
-
-		if ( empty( $extensions[0] ) ) {
+		if ( empty( $mime_type ) ) {
 			return false;
 		}
 
-		return $extensions[0];
+		return wp_get_default_extension_for_mime_type( $mime_type );
 	}
 }
 
diff --git a/src/wp-includes/functions.php b/src/wp-includes/functions.php
index 7b01568ad4..02d74e1b70 100644
--- a/src/wp-includes/functions.php
+++ b/src/wp-includes/functions.php
@@ -2488,6 +2488,10 @@ function wp_unique_filename( $dir, $filename, $unique_filename_callback = null )
 	$filename = sanitize_file_name( $filename );
 	$ext2     = null;
 
+	// Initialize vars used in the wp_unique_filename filter.
+	$number        = '';
+	$alt_filenames = array();
+
 	// Separate the filename into a name and extension.
 	$ext  = pathinfo( $filename, PATHINFO_EXTENSION );
 	$name = pathinfo( $filename, PATHINFO_BASENAME );
@@ -2508,8 +2512,7 @@ function wp_unique_filename( $dir, $filename, $unique_filename_callback = null )
 	if ( $unique_filename_callback && is_callable( $unique_filename_callback ) ) {
 		$filename = call_user_func( $unique_filename_callback, $dir, $name, $ext );
 	} else {
-		$number = '';
-		$fname  = pathinfo( $filename, PATHINFO_FILENAME );
+		$fname = pathinfo( $filename, PATHINFO_FILENAME );
 
 		// Always append a number to file names that can potentially match image sub-size file names.
 		if ( $fname && preg_match( '/-(?:\d+x\d+|scaled|rotated)$/', $fname ) ) {
@@ -2519,37 +2522,54 @@ function wp_unique_filename( $dir, $filename, $unique_filename_callback = null )
 			$filename = str_replace( "{$fname}{$ext}", "{$fname}-{$number}{$ext}", $filename );
 		}
 
-		// Change '.ext' to lower case.
-		if ( $ext && strtolower( $ext ) != $ext ) {
-			$ext2      = strtolower( $ext );
-			$filename2 = preg_replace( '|' . preg_quote( $ext ) . '$|', $ext2, $filename );
+		// Get the mime type. Uploaded files were already checked with wp_check_filetype_and_ext()
+		// in _wp_handle_upload(). Using wp_check_filetype() would be sufficient here.
+		$file_type = wp_check_filetype( $filename );
+		$mime_type = $file_type['type'];
 
-			// Check for both lower and upper case extension or image sub-sizes may be overwritten.
-			while ( file_exists( $dir . "/{$filename}" ) || file_exists( $dir . "/{$filename2}" ) ) {
-				$new_number = (int) $number + 1;
-				$filename   = str_replace( array( "-{$number}{$ext}", "{$number}{$ext}" ), "-{$new_number}{$ext}", $filename );
-				$filename2  = str_replace( array( "-{$number}{$ext2}", "{$number}{$ext2}" ), "-{$new_number}{$ext2}", $filename2 );
-				$number     = $new_number;
+		$is_image    = ( ! empty( $mime_type ) && 0 === strpos( $mime_type, 'image/' ) );
+		$upload_dir  = wp_get_upload_dir();
+		$lc_filename = null;
+
+		$lc_ext = strtolower( $ext );
+		$_dir   = trailingslashit( $dir );
+
+		// If the extension is uppercase add an alternate file name with lowercase extension. Both need to be tested
+		// for uniqueness as the extension will be changed to lowercase for better compatibility with different filesystems.
+		// Fixes an inconsistency in WP < 2.9 where uppercase extensions were allowed but image sub-sizes were created with
+		// lowercase extensions.
+		if ( $ext && $lc_ext !== $ext ) {
+			$lc_filename = preg_replace( '|' . preg_quote( $ext ) . '$|', $lc_ext, $filename );
+		}
+
+		// Increment the number added to the file name if there are any files in $dir whose names match one of the
+		// possible name variations.
+		while ( file_exists( $_dir . $filename ) || ( $lc_filename && file_exists( $_dir . $lc_filename ) ) ) {
+			$new_number = (int) $number + 1;
+
+			if ( $lc_filename ) {
+				$lc_filename = str_replace( array( "-{$number}{$lc_ext}", "{$number}{$lc_ext}" ), "-{$new_number}{$lc_ext}", $lc_filename );
 			}
 
-			$filename = $filename2;
-		} else {
-			while ( file_exists( $dir . "/{$filename}" ) ) {
-				$new_number = (int) $number + 1;
-
-				if ( '' === "{$number}{$ext}" ) {
-					$filename = "{$filename}-{$new_number}";
-				} else {
-					$filename = str_replace( array( "-{$number}{$ext}", "{$number}{$ext}" ), "-{$new_number}{$ext}", $filename );
-				}
-
-				$number = $new_number;
+			if ( '' === "{$number}{$ext}" ) {
+				$filename = "{$filename}-{$new_number}";
+			} else {
+				$filename = str_replace( array( "-{$number}{$ext}", "{$number}{$ext}" ), "-{$new_number}{$ext}", $filename );
 			}
+
+			$number = $new_number;
+		}
+
+		// Change the extension to lowercase if needed.
+		if ( $lc_filename ) {
+			$filename = $lc_filename;
 		}
 
 		// Prevent collisions with existing file names that contain dimension-like strings
 		// (whether they are subsizes or originals uploaded prior to #42437).
-		$upload_dir = wp_get_upload_dir();
+
+		$files = array();
+		$count = 10000;
 
 		// The (resized) image files would have name and extension, and will be in the uploads dir.
 		if ( $name && $ext && @is_dir( $dir ) && false !== strpos( $dir, $upload_dir['basedir'] ) ) {
@@ -2579,18 +2599,77 @@ function wp_unique_filename( $dir, $filename, $unique_filename_callback = null )
 			}
 
 			if ( ! empty( $files ) ) {
-				// The extension case may have changed above.
-				$new_ext = ! empty( $ext2 ) ? $ext2 : $ext;
+				$count = count( $files );
 
 				// Ensure this never goes into infinite loop
 				// as it uses pathinfo() and regex in the check, but string replacement for the changes.
-				$count = count( $files );
-				$i     = 0;
+				$i = 0;
 
 				while ( $i <= $count && _wp_check_existing_file_names( $filename, $files ) ) {
 					$new_number = (int) $number + 1;
-					$filename   = str_replace( array( "-{$number}{$new_ext}", "{$number}{$new_ext}" ), "-{$new_number}{$new_ext}", $filename );
-					$number     = $new_number;
+
+					// If $ext is uppercase it was replaced with the lowercase version after the previous loop.
+					$filename = str_replace( array( "-{$number}{$lc_ext}", "{$number}{$lc_ext}" ), "-{$new_number}{$lc_ext}", $filename );
+
+					$number = $new_number;
+					$i++;
+				}
+			}
+		}
+
+		// Check if an image will be converted after uploading or some existing images sub-sizes file names may conflict
+		// when regenerated. If yes, ensure the new file name will be unique and will produce unique sub-sizes.
+		if ( $is_image ) {
+			$output_formats = apply_filters( 'image_editor_output_format', array(), $_dir . $filename, $mime_type );
+			$alt_types      = array();
+
+			if ( ! empty( $output_formats[ $mime_type ] ) ) {
+				// The image will be converted to this format/mime type.
+				$alt_mime_type = $output_formats[ $mime_type ];
+
+				// Other types of images whose names may conflict if their sub-sizes are regenerated.
+				$alt_types = array_keys( array_intersect( $output_formats, array( $mime_type, $alt_mime_type ) ) );
+				$alt_types[] = $alt_mime_type;
+			} elseif ( ! empty( $output_formats ) ) {
+				$alt_types = array_keys( array_intersect( $output_formats, array( $mime_type ) ) );
+			}
+
+			// Remove duplicates and the original mime type. It will be added later if needed.
+			$alt_types = array_unique( array_diff( $alt_types, array( $mime_type ) ) );
+
+			foreach ( $alt_types as $alt_type ) {
+				$alt_ext = wp_get_default_extension_for_mime_type( $alt_type );
+
+				if ( ! $alt_ext ) {
+					continue;
+				}
+
+				$alt_ext      = ".{$alt_ext}";
+				$alt_filename = preg_replace( '|' . preg_quote( $lc_ext ) . '$|', $alt_ext, $filename );
+
+				$alt_filenames[ $alt_ext ] = $alt_filename;
+			}
+
+			if ( ! empty( $alt_filenames ) ) {
+				// Add the original filename. It needs to be checked again together with the alternate filenames
+				// when $number is incremented.
+				$alt_filenames[ $lc_ext ] = $filename;
+
+				// Ensure no infinite loop.
+				$i = 0;
+
+				while ( $i <= $count && _wp_check_alternate_file_names( $alt_filenames, $_dir, $files ) ) {
+					$new_number = (int) $number + 1;
+
+					foreach ( $alt_filenames as $alt_ext => $alt_filename ) {
+						$alt_filenames[ $alt_ext ] = str_replace( array( "-{$number}{$alt_ext}", "{$number}{$alt_ext}" ), "-{$new_number}{$alt_ext}", $alt_filename );
+					}
+
+					// Also update the $number in (the output) $filename.
+					// If the extension was uppercase it was already replaced with the lowercase version.
+					$filename = str_replace( array( "-{$number}{$lc_ext}", "{$number}{$lc_ext}" ), "-{$new_number}{$lc_ext}", $filename );
+
+					$number = $new_number;
 					$i++;
 				}
 			}
@@ -2601,13 +2680,42 @@ function wp_unique_filename( $dir, $filename, $unique_filename_callback = null )
 	 * Filters the result when generating a unique file name.
 	 *
 	 * @since 4.5.0
+	 * @since 5.8.1 The `$alt_filenames` and `$number` parameters were added.
 	 *
 	 * @param string        $filename                 Unique file name.
 	 * @param string        $ext                      File extension, eg. ".png".
 	 * @param string        $dir                      Directory path.
 	 * @param callable|null $unique_filename_callback Callback function that generates the unique file name.
+	 * @param string[]      $alt_filenames            Array of alternate file names that were checked for collisions.
+	 * @param int|string    $number                   The highest number that was used to make the file name unique
+	 *                                                or an empty string if unused.
 	 */
-	return apply_filters( 'wp_unique_filename', $filename, $ext, $dir, $unique_filename_callback );
+	return apply_filters( 'wp_unique_filename', $filename, $ext, $dir, $unique_filename_callback, $alt_filenames, $number );
+}
+
+/**
+ * Helper function to test if each of an array of file names could conflict with existing files.
+ *
+ * @since 5.8.1
+ * @access private
+ *
+ * @param string[] $filenames Array of file names to check.
+ * @param string   $dir       The directory containing the files.
+ * @param array    $files     An array of existing files in the directory. May be empty.
+ * @return bool True if the tested file name could match an existing file, false otherwise.
+ */
+function _wp_check_alternate_file_names( $filenames, $dir, $files ) {
+	foreach ( $filenames as $filename ) {
+		if ( file_exists( $dir . $filename ) ) {
+			return true;
+		}
+
+		if ( ! empty( $files ) && _wp_check_existing_file_names( $filename, $files ) ) {
+			return true;
+		}
+	}
+
+	return false;
 }
 
 /**
@@ -2793,6 +2901,26 @@ function wp_ext2type( $ext ) {
 	}
 }
 
+/**
+ * Returns first matched extension for the mime-type,
+ * as mapped from wp_get_mime_types().
+ *
+ * @since 5.8.1
+ *
+ * @param string $mime_type
+ *
+ * @return string|false
+ */
+function wp_get_default_extension_for_mime_type( $mime_type ) {
+	$extensions = explode( '|', array_search( $mime_type, wp_get_mime_types(), true ) );
+
+	if ( empty( $extensions[0] ) ) {
+		return false;
+	}
+
+	return $extensions[0];
+}
+
 /**
  * Retrieve the file type from the file name.
  *
diff --git a/tests/phpunit/data/images/test-image-1-100x100.jpg b/tests/phpunit/data/images/test-image-1-100x100.jpg
new file mode 100644
index 0000000000..534aac1d6b
Binary files /dev/null and b/tests/phpunit/data/images/test-image-1-100x100.jpg differ
diff --git a/tests/phpunit/data/images/test-image-2.gif b/tests/phpunit/data/images/test-image-2.gif
new file mode 100644
index 0000000000..8fad364479
Binary files /dev/null and b/tests/phpunit/data/images/test-image-2.gif differ
diff --git a/tests/phpunit/data/images/test-image-3.jpg b/tests/phpunit/data/images/test-image-3.jpg
new file mode 100644
index 0000000000..534aac1d6b
Binary files /dev/null and b/tests/phpunit/data/images/test-image-3.jpg differ
diff --git a/tests/phpunit/data/images/test-image-4.png b/tests/phpunit/data/images/test-image-4.png
new file mode 100644
index 0000000000..642ce92975
Binary files /dev/null and b/tests/phpunit/data/images/test-image-4.png differ
diff --git a/tests/phpunit/tests/functions.php b/tests/phpunit/tests/functions.php
index 4ef24d22a1..cdeb2361a8 100644
--- a/tests/phpunit/tests/functions.php
+++ b/tests/phpunit/tests/functions.php
@@ -168,13 +168,21 @@ class Tests_Functions extends WP_UnitTestCase {
 		$testdir = DIR_TESTDATA . '/images/';
 
 		// Sanity check.
-		$this->assertSame( 'abcdefg.png', wp_unique_filename( $testdir, 'abcdefg.png' ), 'Sanitiy check failed' );
+		$this->assertSame( 'abcdefg.png', wp_unique_filename( $testdir, 'abcdefg.png' ), 'Test non-existing file, file name should be unchanged.' );
 
-		// Check number is appended for file already exists.
+		// Ensure correct images exist.
 		$this->assertFileExists( $testdir . 'test-image.png', 'Test image does not exist' );
-		$this->assertSame( 'test-image-1.png', wp_unique_filename( $testdir, 'test-image.png' ), 'Number not appended correctly' );
 		$this->assertFileDoesNotExist( $testdir . 'test-image-1.png' );
 
+		// Check number is appended if file already exists.
+		$this->assertSame( 'test-image-1.png', wp_unique_filename( $testdir, 'test-image.png' ), 'File name not unique, number not appended.' );
+
+		// Check file with uppercase extension.
+		$this->assertSame( 'test-image-1.png', wp_unique_filename( $testdir, 'test-image.PNG' ), 'File name with uppercase extension not unique, number not appended.' );
+
+		// Check file name with already added number.
+		$this->assertSame( 'test-image-2-1.gif', wp_unique_filename( $testdir, 'test-image-2.gif' ), 'File name not unique, number not appended correctly.' );
+
 		// Check special chars.
 		$this->assertSame( 'testtest-image.png', wp_unique_filename( $testdir, 'testtést-imagé.png' ), 'Filename with special chars failed' );
 
@@ -221,6 +229,88 @@ class Tests_Functions extends WP_UnitTestCase {
 		return $upload_dir;
 	}
 
+	/**
+	 * @ticket 53668
+	 */
+	function test_wp_unique_filename_with_additional_image_extension() {
+		$testdir = DIR_TESTDATA . '/images/';
+
+		add_filter( 'upload_dir', array( $this, 'upload_dir_patch_basedir' ) );
+
+		// Set conversions for uploaded images.
+		add_filter( 'image_editor_output_format', array( $this, 'image_editor_output_format_handler' ) );
+
+		// Ensure the test images exist.
+		$this->assertFileExists( $testdir . 'test-image-1-100x100.jpg', 'test-image-1-100x100.jpg does not exist' );
+		$this->assertFileExists( $testdir . 'test-image-2.gif', 'test-image-2.gif does not exist' );
+		$this->assertFileExists( $testdir . 'test-image-3.jpg', 'test-image-3.jpg does not exist' );
+		$this->assertFileExists( $testdir . 'test-image-4.png', 'test-image-4.png does not exist' );
+
+		// Standard test: file does not exist and there are no possible intersections with other files.
+		$this->assertSame(
+			'abcdef.png',
+			wp_unique_filename( $testdir, 'abcdef.png' ),
+			'The abcdef.png, abcdef.gif, and abcdef.jpg images do not exist. The file name should not be changed.'
+		);
+
+		// Actual clash recognized.
+		$this->assertSame(
+			'canola-1.jpg',
+			wp_unique_filename( $testdir, 'canola.jpg' ),
+			'The canola.jpg image exists. The file name should be unique.'
+		);
+
+		// Same name with different extension and the image will be converted.
+		$this->assertSame(
+			'canola-1.png',
+			wp_unique_filename( $testdir, 'canola.png' ),
+			'The canola.jpg image exists. Uploading canola.png that will be converted to canola.jpg should produce unique file name.'
+		);
+
+		// Same name with different uppercase extension and the image will be converted.
+		$this->assertSame(
+			'canola-1.png',
+			wp_unique_filename( $testdir, 'canola.PNG' ),
+			'The canola.jpg image exists. Uploading canola.PNG that will be converted to canola.jpg should produce unique file name.'
+		);
+
+		// Actual clash with several images with different extensions.
+		$this->assertSame(
+			'test-image-5.png',
+			wp_unique_filename( $testdir, 'test-image.png' ),
+			'The test-image.png, test-image-1-100x100.jpg, test-image-2.gif, test-image-3.jpg, and test-image-4.png images exist.' .
+			'All of them may clash when creating sub-sizes or regenerating thumbnails in the future. The filename should be unique.'
+		);
+
+		// Possible clash with regenerated thumbnails in the future.
+		$this->assertSame(
+			'codeispoetry-1.jpg',
+			wp_unique_filename( $testdir, 'codeispoetry.jpg' ),
+			'The codeispoetry.png image exists. When regenerating thumbnails for it they will be converted to JPG.' .
+			'The name of the newly uploaded codeispoetry.jpg should be made unique.'
+		);
+
+		remove_filter( 'image_editor_output_format', array( $this, 'image_editor_output_format_handler' ) );
+		remove_filter( 'upload_dir', array( $this, 'upload_dir_patch_basedir' ) );
+	}
+
+	/**
+	 * Changes the output format when editing images. When uploading a PNG file
+	 * it will be converted to JPEG, GIF to JPEG, and PICT to BMP
+	 * (if the image editor in PHP supports it).
+	 *
+	 * @param array $formats
+	 *
+	 * @return array
+	 */
+	public function image_editor_output_format_handler( $formats ) {
+		$formats['image/png'] = 'image/jpeg';
+		$formats['image/gif'] = 'image/jpeg';
+		$formats['image/pct'] = 'image/bmp';
+
+		return $formats;
+	}
+
 	/**
 	 * @dataProvider data_is_not_serialized
 	 */
@@ -1946,4 +2036,18 @@ class Tests_Functions extends WP_UnitTestCase {
 			array( 'application/activity+json, application/nojson', true ),
 		);
 	}
+
+	/**
+	 * @ticket 53668
+	 */
+	public function test_wp_get_default_extension_for_mime_type() {
+		$this->assertEquals( 'jpg', wp_get_default_extension_for_mime_type( 'image/jpeg' ), 'jpg not returned as default extension for "image/jpeg"' );
+		$this->assertNotEquals( 'jpeg', wp_get_default_extension_for_mime_type( 'image/jpeg' ), 'jpeg should not be returned as default extension for "image/jpeg"' );
+		$this->assertEquals( 'png', wp_get_default_extension_for_mime_type( 'image/png' ), 'png not returned as default extension for "image/png"' );
+		$this->assertFalse( wp_get_default_extension_for_mime_type( 'wibble/wobble' ), 'false not returned for unrecognized mime type' );
+		$this->assertFalse( wp_get_default_extension_for_mime_type( '' ), 'false not returned when empty string as mime type supplied' );
+		$this->assertFalse( wp_get_default_extension_for_mime_type( '   ' ), 'false not returned when empty string as mime type supplied' );
+		$this->assertFalse( wp_get_default_extension_for_mime_type( 123 ), 'false not returned when int as mime type supplied' );
+		$this->assertFalse( wp_get_default_extension_for_mime_type( null ), 'false not returned when null as mime type supplied' );
+	}
 }