File upload - type is not allowed

[eluser]predat0r[/eluser]
One of my customer has problem with file uploads, especially with .doc files. Sometimes he can't upload files from his office - gets file type is not allowed message - , but when he tries at home, the process is successful. If he sends me the file to upload by me, I always do it without any problem. I updated the mimes.php to the latest types for word documents of course (found here some forum posts about that).
But, sometimes he can upload other .doc files. So where is the error, on server/intranet, upload php script, mime types? If the script doesn't change, the mimes are extended, why this happens sometimes? And why I can upload 10 docs from 10?
Thanks