Fix media upload in MatrixHttpApi

[lordgurke]

Description

Due to a bug, media uploads (i.e. images, audio files) were not working, as the content would always be handled as a JSON file and binary data would break the JSON linter.
This fix also changes the way file contents (if not JSON) are uploaded with Guzzle, as it now is transfered as HTTP body (as per matrix.org API documentation).

How has this been tested?

This was taken out of a working live system patch.

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