This function registers a rich text deserialization function along with its mime_type with the passed buffer.
the format’s mime-type
the deserialize function to register
function’s user_data
a function to call when user_data is no longer needed
the gdk.Atom that corresponds to the newly registered format’s mime-type.
2.10
This function registers a rich text deserialization function along with its mime_type with the passed buffer.