Spreadsheet download

I found this old thread discussing the format of the spreadsheet download:

http://www.gossamer-threads.com/lists/rt/users/58799#58799

Where Jesse says:

Nope. TSV is a perfectly servicable spreadsheet format. (Though we mark 
it as an excel file, so the right thing happens when end-users click on 
it)

Now, I’ve noticed some strange behavior with this. In IE8, the
downloaded file is a .xls, in Chrome the file is a .tsv.xls, and in
Firefox it downloads as a .tsv.

Questions:

  1. How is this extension change happening, and how can I modify it?
  2. Is this a bug and should I report it as such?
  3. Shouldn’t the downloaded file’s extension match what it actually is,
    and not try to help the OS know what it should open in? Excel throws an
    (easily skipped) error up when trying to open the .xls files anyway,
    complaining that it’s not in the format it claims to be.

-Mike