If you'r using 11.10 i found this for an application called nedit.
Quoting mc4man
fairly easy - nedit's .desktop is missing something to allow it to be shown in the r. click context menu
Code:
gksudo gedit /usr/share/applications/nedit.desktop
find the Exec= line & add a %f to end, leave a space between nedit & %f - ignore the untitled document 1 that also opens - close it without saving, do save the nedit.desktop
Code:
[Desktop Entry]
Version=1.0
Name=NEdit
Exec=nedit %f
Icon=Nedit
Terminal=false
Type=Application
Categories=Motif;Utility;TextTools;
GenericName=Text editor
GenericName[en]=Text editor
GenericName[nl]=Tekst verwerker
Then you'll find it by right clicking on your file > open with > other applications
After that it will be shown directly in the r. click menu
If you wish to set nedit as default for all text files then use the right click > Properties > open with instead
If, in other hand, you're using 12.04 you can do it like this:
If you want to delete applications from the openwith list you have to edit the following file from command line
/usr/share/applications/mimeinfo.cache
If you added item by yourself, or you want to add new item you need to edit the following file from command line
~/.local/share/applications/mimeinfo.cache
default actions you need to edit the following file from command line
~/.local/share/applications/defaults.list