Source

File: wp-includes/ms-functions.php

function check_upload_mimes( $mimes ) {
	$site_exts  = explode( ' ', get_site_option( 'upload_filetypes', 'jpg jpeg png gif' ) );
	$site_mimes = array();
	foreach ( $site_exts as $ext ) {
		foreach ( $mimes as $ext_pattern => $mime ) {
			if ( '' !== $ext && false !== strpos( $ext_pattern, $ext ) ) {
				$site_mimes[ $ext_pattern ] = $mime;
			}
		}
	}
	return $site_mimes;
}