Allow CSS Files Upload

/**

 * Allow CSS uploads for administrator users.

 *

 * @param array $upload_mimes Allowed mime types.

 *

 * @return mixed

 */

add_filter(

	'upload_mimes',

	function ( $upload_mimes ) {

		// By default, only administrator users are allowed to add SVGs.

		// To enable more user types edit or comment the lines below but beware of

		// the security risks if you allow any user to upload SVG files.

		if ( ! current_user_can( 'administrator' ) ) {

			return $upload_mimes;

		}

		$upload_mimes['css']  = 'text/css';

		return $upload_mimes;

	}

);

/**

 * Add CSS files mime check.

 *

 * @param array        $wp_check_filetype_and_ext Values for the extension, mime type, and corrected filename.

 * @param string       $file Full path to the file.

 * @param string       $filename The name of the file (may differ from $file due to $file being in a tmp directory).

 * @param string[]     $mimes Array of mime types keyed by their file extension regex.

 * @param string|false $real_mime The actual mime type or false if the type cannot be determined.

 */

add_filter(

	'wp_check_filetype_and_ext',

	function ( $wp_check_filetype_and_ext, $file, $filename, $mimes, $real_mime ) {

		if ( ! $wp_check_filetype_and_ext['type'] ) {

			$check_filetype  = wp_check_filetype( $filename, $mimes );

			$ext             = $check_filetype['ext'];

			$type            = $check_filetype['type'];

			$proper_filename = $filename;

			if ( $type && 0 === strpos( $type, 'text/' ) && 'css' !== $ext ) {

				$ext  = false;

				$type = false;

			}

			$wp_check_filetype_and_ext = compact( 'ext', 'type', 'proper_filename' );

		}

		return $wp_check_filetype_and_ext;

	},

	10,

	5

);