Add support for multipart-bodies (RFC 2046)

[Kaliumhexacyanoferrat]

As a developer of a web service, I would like to be able to retrieve multipart content so that I can handle it in my application.

Acceptance criteria

• Content can either be handled through a specific type or already on framework layer (so that I just inject a Stream into my webservice method for file uploads)
• The feature is enabled by default
• The feature has been tested with common browsers and clients (such as curl, Postman)
• The feature is covered by acceptance tests

[Kaliumhexacyanoferrat]

In my opinion modern SPAs and APIs are using single requests instead of bundling them into a multipart request as this is easier to develop, debug and maintain. This issue will stay in the backlog and probably be implemented some day, but lost it's priority status for now.