MIME type for XPI files (Firefox addon installers)
 

Webservers (IIS, Apache, etc) require MIME types to understand what certain files are, in order to know how to handle them.
Many applications do, too -- for example, vBulletin requires the MIME type when you add custom entries into the attachments manager.

This is the proper MIME type for a .xpi file:
Hope that info helps somebody. :thumb: