Optimise max file size value converter.
Assume nonces are generated properly.

I am not a security expert, but it seems good enough.
DRY required constraint.

Add some notes.
Check for emptiness of text and file fields correctly and DRY some internal code.

Yeah, and screw half of best OOP practices in the process...
Bring the two type specific validate() implementation into the same format.

Hopefully, there will only be one left by the next commit.
Throw away custom file validation routine.
Rename aeFile::type() to aeFile::extension().
Stop using erroneous "mimetype". It's MIME.
Fix file extension check failing, i.e. jpeg !== jpg.