Networks and Sites: Ensure fileupload_maxk is an int to avoid potential fatal errors.

This changeset fixes a potential fatal error, for example when "Max upload file size" setting is set to an empty value. It also adds unit tests for `upload_size_limit_filter`.

Props mjkhajeh, bhrugesh12, SergeyBiryukov, kebbet, audrasjb, felipeelia.
Fixes #55926.


git-svn-id: https://develop.svn.wordpress.org/trunk@54482 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/formatting.php

@@ -4771,6 +4771,7 @@ function sanitize_option( $option, $value ) {
 		case 'users_can_register':
 		case 'start_of_week':
 		case 'site_icon':
+		case 'fileupload_maxk':
 			$value = absint( $value );
 			break;