Mime::Type.register "image/jpg", :jpg, ["image/jpeg", "image/pjpeg"]
Above example uses mime type synonyms. Each synonym is a alternative
string used to identify the same mime type and appears as an array
after the mime type symbol.
The class document does not explain the meaning of mime_type_synonyms
nor give an example. Someone should fix that.
Hope this post clarifies use of mime type synonyms.