Basically what this is doing is saying "Are you a Mac?"
If the answer is "Yes" then it says "fine, Macs don't let you manually type a path so whatever you have must be fine. Approved."
If the answer is "No" then it must assume that you are a Windows machine and checks to see if your path conforms to Windows norms. A path like "/home/wedge/mydoc.doc" looks to be invalid so it refuses to take it.
The key to the workaround that I found is that it doesn't even check the path if it thinks you are a Mac. To detirmine if you are a Mac it just checks for the presense of the string "mac" (not case sensitive) in the user agent string that your browser sends. So you can get a user agent switcher (available
here for Firefox, should be just a couple clicks to get it installed). Now just create a new user agent and just put the word "mac" on the line for user agent. Then when you go to upload your file just switch to that custom one and it will think you are a mac and accept your file.