diff --git a/src/wp-admin/includes/file.php b/src/wp-admin/includes/file.php
index 759b0974bb..ddd2be849a 100644
--- a/src/wp-admin/includes/file.php
+++ b/src/wp-admin/includes/file.php
@@ -2555,41 +2555,3 @@ function wp_opcache_invalidate( $filepath, $force = false ) {
 
 	return false;
 }
-
-/**
- * Wrapper for PHP filesize with filters and casting the result as an integer.
- *
- * @since 6.0.0
- *
- * @link https://www.php.net/manual/en/function.filesize.php
- *
- * @param string $path Path to the file.
- * @return int The size of the file in bytes, or 0 in the event of an error.
- */
-function wp_filesize( $path ) {
-	/**
-	 * Filters the result of wp_filesize before the PHP function is run.
-	 *
-	 * @since 6.0.0
-	 *
-	 * @param null|int $size The unfiltered value. Returning an int from the callback bypasses the filesize call.
-	 * @param string   $path Path to the file.
-	 */
-	$size = apply_filters( 'pre_wp_filesize', null, $path );
-
-	if ( is_int( $size ) ) {
-		return $size;
-	}
-
-	$size = (int) @filesize( $path );
-
-	/**
-	 * Filters the size of the file.
-	 *
-	 * @since 6.0.0
-	 *
-	 * @param int    $size The result of PHP filesize on the file.
-	 * @param string $path Path to the file.
-	 */
-	return (int) apply_filters( 'wp_filesize', $size, $path );
-}
diff --git a/src/wp-includes/functions.php b/src/wp-includes/functions.php
index a2c1650c49..a93285c5fc 100644
--- a/src/wp-includes/functions.php
+++ b/src/wp-includes/functions.php
@@ -3458,6 +3458,44 @@ function wp_get_ext_types() {
 	);
 }
 
+/**
+ * Wrapper for PHP filesize with filters and casting the result as an integer.
+ *
+ * @since 6.0.0
+ *
+ * @link https://www.php.net/manual/en/function.filesize.php
+ *
+ * @param string $path Path to the file.
+ * @return int The size of the file in bytes, or 0 in the event of an error.
+ */
+function wp_filesize( $path ) {
+	/**
+	 * Filters the result of wp_filesize before the PHP function is run.
+	 *
+	 * @since 6.0.0
+	 *
+	 * @param null|int $size The unfiltered value. Returning an int from the callback bypasses the filesize call.
+	 * @param string   $path Path to the file.
+	 */
+	$size = apply_filters( 'pre_wp_filesize', null, $path );
+
+	if ( is_int( $size ) ) {
+		return $size;
+	}
+
+	$size = (int) @filesize( $path );
+
+	/**
+	 * Filters the size of the file.
+	 *
+	 * @since 6.0.0
+	 *
+	 * @param int    $size The result of PHP filesize on the file.
+	 * @param string $path Path to the file.
+	 */
+	return (int) apply_filters( 'wp_filesize', $size, $path );
+}
+
 /**
  * Retrieve list of allowed mime types and file extensions.
  *
diff --git a/tests/phpunit/tests/file.php b/tests/phpunit/tests/file.php
index cf3ec2d356..311ba30339 100644
--- a/tests/phpunit/tests/file.php
+++ b/tests/phpunit/tests/file.php
@@ -262,39 +262,4 @@ class Tests_File extends WP_UnitTestCase {
 
 		return $keys;
 	}
-
-	/**
-	 * @ticket 49412
-	 * @covers ::wp_filesize
-	 */
-	function test_wp_filesize_with_nonexistent_file() {
-		$file = 'nonexistent/file.jpg';
-		$this->assertEquals( 0, wp_filesize( $file ) );
-	}
-
-	/**
-	 * @ticket 49412
-	 * @covers ::wp_filesize
-	 */
-	function test_wp_filesize() {
-		$file = DIR_TESTDATA . '/images/test-image-upside-down.jpg';
-
-		$this->assertEquals( filesize( $file ), wp_filesize( $file ) );
-
-		$filter = function() {
-			return 999;
-		};
-
-		add_filter( 'wp_filesize', $filter );
-
-		$this->assertEquals( 999, wp_filesize( $file ) );
-
-		$pre_filter = function() {
-			return 111;
-		};
-
-		add_filter( 'pre_wp_filesize', $pre_filter );
-
-		$this->assertEquals( 111, wp_filesize( $file ) );
-	}
 }
diff --git a/tests/phpunit/tests/functions.php b/tests/phpunit/tests/functions.php
index 96dee380ec..294a642f86 100644
--- a/tests/phpunit/tests/functions.php
+++ b/tests/phpunit/tests/functions.php
@@ -2106,4 +2106,39 @@ class Tests_Functions extends WP_UnitTestCase {
 		$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' );
 	}
+
+	/**
+	 * @ticket 49412
+	 * @covers ::wp_filesize
+	 */
+	function test_wp_filesize_with_nonexistent_file() {
+		$file = 'nonexistent/file.jpg';
+		$this->assertEquals( 0, wp_filesize( $file ) );
+	}
+
+	/**
+	 * @ticket 49412
+	 * @covers ::wp_filesize
+	 */
+	function test_wp_filesize() {
+		$file = DIR_TESTDATA . '/images/test-image-upside-down.jpg';
+
+		$this->assertEquals( filesize( $file ), wp_filesize( $file ) );
+
+		$filter = function() {
+			return 999;
+		};
+
+		add_filter( 'wp_filesize', $filter );
+
+		$this->assertEquals( 999, wp_filesize( $file ) );
+
+		$pre_filter = function() {
+			return 111;
+		};
+
+		add_filter( 'pre_wp_filesize', $pre_filter );
+
+		$this->assertEquals( 111, wp_filesize( $file ) );
+	}
 }