Modify HTTP Response Headers. Actually only Content-Type is modified. The primary use is to force a specific content-type when servers send application/octet-stream. This makes it possible to tick "Do this automatically for files like this from now on." for new types of archives, for example.
To force a content-type, change the modifyresponse.rules preference in about:config. It's a "|"-separated list of "url_regex contenttype_regex contenttype_replacement" entries. See the default value for an example.