Multipart data has boundaries which separate various parts. What's happening is that it's running out of data in the request without seeing the ending boundary. That could be because of a broken web browser ignoring specifications, lost packets, elves, or an aborted upload.

It also could be a bug in cgi-lib; standards have changed somewhat in the past several years.

If it's a client side issue (and it's probably a client side issue), the best thing any server component can do is fail gracefully. You can't be sure you've received the whole file, so don't try to process it.