Installing 0.9.31 messes with file types in Ubuntu

I recently upgraded my old 0.9.27 to 0.9.31 (interesting bugfix for my Nook Color) on my old Ubuntu Lucid and not only did calibre stopped working (and was nowhere to be found although install proceeded without a hitch) but it messed with many filetype association.

All epub and mobi and pdf and video files were changed to Text/plain.

I have been trying to revert to 0.9.30 then 0.9.29 and now 0.9.27 following the instructions, but to no avail.

Please advise.