I have a file that is formatted as a .doc, but Nautilus and LibreOffice insists that it is a .txt. Both precise and Ubuntu 12.10 are that way, but Google Docs can convert it (after playing with it).



To check if 'extension' mime type exist:
grep 'extension' /etc/mime.types
To create new mime type:
Open /etc/mime.types with text-editor(with root privileges)
gksudo gedit /etc/mime.types
Add extension to /etc/mime.types in following format:
text/extension extension
Could be something as simple as a mangled/intentionally changed header. Here's what the MS Word first 2 chunks looks like in a couple of documents I just checked:
D0 CF 11 E0 A1 B1 1A E1
You could MAKE A BACKUP and try changing the first several bytes to that signature and see if it doesn't at least TRY to open as the right thing.
Generally, file can look at this area:

And tell the file-type. If it's detecting it wrong, either it actually is a text file renamed to doc or the header got borked.
Properties from the context menu.Properties click on the Open With.writer I asssume). All files with the same extension will now be opened with this program by default.
You can do this manually too
/usr/share/applications/defaults.list holds the defaults (ie. system wide associations).~/.local/share/applications/mimeapps.list holds user specifics.By the way: I assume you have software installed (ie. LibreOffice) that can open doc files?
From the comments and images added:
.txt files to open with Writer (just temporary) and see if it then does open and then save it under another name.
Then reset opening txt to Gedit or what it was before.
text/plaindocument, not as a MS word doc. Removed the +'s, no difference. – jrg Aug 24 '12 at 14:17