Important: An officially mentioned type either does not exist, or is hard to track down. The information here likely reflects community contributions or popular usage derived from existing implementations.
The packaging format for Firefox add-ons. An .xpi file is an ordinary ZIP archive containing the extension's manifest, scripts and resources, and this media type is what tells Firefox to offer to install the file rather than simply download it.
It is not registered with IANA, but Mozilla's install flow depends on it and GitHub's releases API reports it for add-ons published there. Serving an add-on as application/zip instead will stop the browser offering to install it.
File types
.xpi
Also appears as
Not known to appear as any other types