Upload: Correct duplicate MIME type for .xlsx files generated by Google Docs.

This expands the code block previously added for `.docx` files to include `.xlsx` files as well, which are known to have the same issue with `finfo_file()`.

Includes a unit test case for `wp_check_filetype_and_ext()`.

Reference: [https://bugs.php.net/bug.php?id=77784 PHP Bug #77784: mime_content_type() result gets doubled for .xlsx].

Follow-up to [56497].

See #57898.

git-svn-id: https://develop.svn.wordpress.org/trunk@56510 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/functions.php

@@ -1643,6 +1643,16 @@ class Tests_Functions extends WP_UnitTestCase {
 							'proper_filename' => false,
 						),
 					),
+					// Google Docs file for which finfo_file() returns a duplicate mime type.
+					array(
+						DIR_TESTDATA . '/uploads/double-mime-type.docx',
+						'double-mime-type.docx',
+						array(
+							'ext'             => 'docx',
+							'type'            => 'application/vnd.openxmlformats-officedocument.wordprocessingml.document',
+							'proper_filename' => false,
+						),
+					),
 					// Non-image file with wrong sub-type.
 					array(
 						DIR_TESTDATA . '/uploads/pages-to-word.docx',