Upload xpi file of a release with special content-type

[zuphilip]

Currently, when clicking on a XPI file in Firefox, it first tries to install this as a Firefox plugin. Thus, one usually need to right click on the file and save it before installing in Zotero. The idea is to use a special content-type which can then also be handled by the Zotero connector, see also zotero/zotero-connectors#297 .