URI scheme handling (such as a movie player handling mms:// URIs, or a Podcast program handling feed:// URIs) are handled through applications handling the x-scheme-handler/foo mime-type, where foo is the URI scheme in question.
This scheme allows URI scheme handling to enjoy the same benefits as mime-type handlers, such as the ability to change the default handler, the cross-desktop support, and easier application launching.
Note that this virtual mime-type is not for listing URI schemes that an application can load files from. For example, a movie player would not list x-scheme-handler/http in its supported mime-types, but it would list x-scheme-handler/rtsp if it supported playing back from RTSP locations.