I'm trying to figure some of this out myself but I've found you can add items to your menu by creating .desktop files in /usr/share/applications/. For instance I have
Code:
[Desktop Entry]
Encoding=UTF-8
Version=0.0.0
Name=Bittornado
Comment=Bittorrent Download Manager
Exec=btdownloadgui.py
Icon=/usr/share/icons/Rodent/48x48/stock/stock_download.png
Terminal=false
Type=Application
Categories=Application;Network
MimeType=application/bittorrent.bt
The last part is what I dont get. It's defined in /usr/share/xfce4/mime/Rodent.mime.xml so I guess some searching in there can lead to good mimetypes.