[FIX] website_slides: allow upload of >20MB files (as limit is 25MB)

There is a double-check (on client and on server) to prevent the user to
upload a >25 MB document. The client checks the actual file size while
the server guesses it from the total received payload. This server guess
is wrong as the extra payload is quite huge (5-10MB). Now the server
checks the real file size as it should have from the beginning.