Making MIME Types and file associations work universally on Linux is fairly difficult. Here are some tips:

• Install FreeDesktop.org MIME Type .xml files into share/mime/packages. makepackage will automatically generate the KDE equivalents and install them automatically when you install the .xml file with installMime().

• To work around Gnome Bug #309862 you need to add the correct <sub-class-of> entry. For example: <sub-class-of type="text/plain" />
  Basically, use the type that Gnome thinks your file is without your special MIME type installed.

• The filename for your MIME Type icons should be in the format of: gnome-mime-category-name.ext. For example: gnome-mime-application-x-traverso.png for files of type application/x-traverso. The icons should be installed into <theme>/<size>/mimetypes.