I have added reporting of the mime type to the error message. When you attempt to upload an archive, if it fails with 'is not a valid/acceptable archive format' messsage, make a note of the exact reported mime type. And send me a mail (instructions in the 'Bugs' seciton of the competition site), I'll review the problem and if a fix is needed, I'll get it sorted.

As a consequence of testing this, I'm about to add the WinRAR mime type application/x-rar to the list.