Always force server-side attachments validation

osTicket supports filtering allowable files on the client-side via JS as
well as server-side on upload.

Ajax based upload skipped server-side validation with the assumption that
the client already validated the file upload. For most cases this is a valid
assumption (ajax only works if JS is enabled) but fails to account for cases
where HTTP requests is intercepted and changed on transit or the request is
posted directly to the ajax interface.

This commit forces server-side file upload validation.