I have tried to upload several PowerPoint files (.ppt) files using different web browsers and I keep getting the same error:

You must correct the following error(s) before proceeding:
Error uploading your file. Confirm it is a .ppt or .sxi file (or a .zip file containing GIF/JPEG images)

I have verified that the file I'm uploading has the .ppt extension.  Even when I zip the .ppt file and try to upload the zipped file I get the same error message.

Uploading a zip, gif, or jpeg by themselves works fine, but uploading a .ppt file does not work.  The only work around that I found was to save the entire PowerPoint presentation as a GIF or JPEG, zip all the files, and then upload the zip file.