In attempting to tighten our uploads, we also have run into this problem. I was able to work around the problem thanks to this writeup. I made sure that I accepted both excel and msword if the type of file being uploaded was excel. Then after the call to this function, I needed to manually change the file type to excel before continuing the upload.
It works ok, but the bug really should be investigated and fixed. I have no idea what happened to your bug report, but it is no longer accessible.