I tried to open a text file with Ubuntu’s Document Viewer  – turns out it doesn’t read text files.  However, now whenever I go right-click on a text file it gives me the option to “Open with Document Viewer” which I know doesn’t work – and I swear I de-selected the check box asking “Remember this application for…”.

Anyway – to fix it I found a great post at http://ubuntuforums.org/archive/index.php/t-51012.html – here’s an abridged version of the bits I needed:

The myth of gnome 2.10 is that the “Open With” program will automatically do file associations for you. Well this is a complete LIE. There’s a lot of manipulatations needed to get the currect applications opening with a specified file type. Lets jump in!!

[ Gnome 2.10 uses a couple of ways to store file associations. ]1.) /usr/share/applications/defaults.list– This method is perhaps one of the most important. Following this comes..
2.) /usr/share/applications/mimeinfo.cache– This is the cache of the current mime types. Updating the mime database doesn’t work to well so of course you have to manually edit this as well.
3.) ~/.local/share/applications/– The dreaded “Open With”
4.) ~/.gnome/share/apps/– Same as the above

Thanks to FLeiXiuS