According to an article found on internet via Google search (Google is your friend) ...
associated with a file upload attempt, and can occur when:
* Attempting to upload a file with the same name as another already-uploaded file;
* Attempting to upload a file that is too large for the system
(Sitewide the limit is XMB; however, individual classes may vary — check your course settings)
* Attempting to upload a file type that the system doesn’t recognize; or
* Attempting to upload a file with a period or special characters in the file name.
'spirit of sharing', Ken