UploadedMedia: Add missing disposition_type to Content-Disposition

Set it to `inline` because the vast majority of what's sent is multimedia
content while `attachment` would have the side-effect of triggering a
download dialog.

Closes: https://git.pleroma.social/pleroma/pleroma/-/issues/3114

test/pleroma/web/plugs/uploaded_media_plug_test.exs

@@ -37,7 +37,7 @@ defmodule Pleroma.Web.Plugs.UploadedMediaPlugTest do
 
     assert Enum.any?(
              conn.resp_headers,
-             &(&1 == {"content-disposition", "filename=\"\\\"cofe\\\".gif\""})
+             &(&1 == {"content-disposition", "inline; filename=\"\\\"cofe\\\".gif\""})
            )
   end
 end