What's the size of each of the files?
I'd say 99% of PHP installations have a max-post size of 8Mb, but only 2Mb for post-files. If your .psd file (or any other file for that matter) is over that, it's an automatic fail.
It's easy to change if you have your own copy of PHP running - you just change the values in the php.ini file. Otherwise, speak to your systems admin and they should be able to fix that for you.
That is, of course, if that IS the problem. Can you post a code snippet to determine if there's anything out of sorts? The enctype is definitely "multipart/form-data"? Sometimes a hidden field within the form called "MAX_FILE_SIZE" with a value of "4096000" may help.