Clarify file formats and use of Content-Type headers

[philipashlock]

The use of format terms like "XML" and "JSON" can be ambiguous without specifying HTTP Content-Type headers. I would also recommend specifying the character encoding here, but I'll mention that as another issue. I would recommend XML and JSON usage as the following types and the guidelines should articulate that HTTP headers should match this for each format.

XML: Content-Type: text/xml; charset=utf-8
JSON: Content-Type: application/json; charset=utf-8