Okay, according to the response headers, the file was being uploaded successfully, but the uploader-form never acknowledged this. It works properly for files that have been designated as image files, since the problem is that the Javascript is looking for a thumbnail when there is none. Additionally, the problem is that image file-types were not properly flagged as images when VaultWiki was installed.
This is all fixed now, but you will have to go through Content > Attachments, and make sure all the appropriate types have "Is this an Image type?" So that thumbnails will be made for those types.