http://ubuntuforums.org/showthread.php?t=2003610


Add MIME type
===========================


<?xml version='1.0' encoding='utf-8'?>
<mime-info xmlns="http://www.freedesktop.org/standards/shared-mime-info">
    <mime-type type="application/rails">
        <comment>Rails</comment>
        <glob pattern="*.rails"/>
    </mime-type>
</mime-info>


sudo xdg-mime install --novendor  ~/Documents/rails.xml

sudo update-mime-database   /usr/share/mime

sudo gedit  /usr/share/applications/rails.desktop

In the .desktop file:

    MimeType=application/rails
    
sudo xdg-desktop-menu install --novendor /usr/share/applications/rails.desktop

sudo xdg-icon-resource install --context mimetypes --size 128 freshmemory.png application-x-fm-dictionary