add the ability to specify an extension other than mime-type

[derVedro]

I add an optional extension key-value pair to WRITE_FORMATS dicts, so it's possible to specify an extension other than the mime type (like JPEG for mime type and .jpg for extension). If no extension key-value pair is provided, the mime type name will be used as extension as before.

Second, I renamed parameter mime_output to format. I think this is a more general name for that dict.

[derVedro]

I think I'm done with the updates for now, I like everything.