For compatibility with existing library consumers, the File field is retained but will behave as if Files contained that single file. If both are specified, ChannelMessageSendComplex will return an error.
The message JSON payload is moved to a form-data field called `payload_json`, instead of set in multipart form data. This is supported and the recommended way, as per the API docs.
Apparently, you can attach multiple files if you just name the parts names differently in the multipart request. The parts are named here using the order the files were specified, as `file%d`. This is not documented in the API docs, but definitely works.
This also removes serialization of the File field via json.Marshal, as it will never be directly serialized in the JSON. The new field, Files, is similarly not marshaled.
This additionally adds a ContentType field in File, which can be used to specify the content type of the attached file. The ContentType field will default to setting the header to `application/octet-stream` if empty. Discord currently doesn't do much with the Content-Type header, but we should pass this information along anyway in accordance to the MIME standard.
