I get that same error trying to upload an image file using Firefox 27.0.1. The error does not occur using IE or Chrome.
To fix the problem: un-check "Use the Flash-based uploader to upload attachments" in Preferences
Or use this workaround, here is the sequence:
Click "More Options" to begin
Click "Upload a File" and then choose a file to upload from the file browse window, click "Open" and this error is returned.
After closing the error message window, this screen appears. Note "Attached Files:" under the 3 orange buttons. This did not appear on the prior screen. Click on "Attached Files:"
and this window displays. Click "Browse" to find the file to upload. The file browse window will open, select a file and click "Open"
The attached file will appear in the list. To add more files, click on "Attached Files:" and repeat.