Specify `Content-Type` and `Content-Disposition` usage in the media repo, as per MSC2701 and MSC2702. (#1935)

Folgende Änderung muss für TI-M behandelt werden:

> - Specify `Content-Type` and `Content-Disposition` usage in the media repo, as per [MSC2701](https://github.com/matrix-org/matrix-spec-proposals/pull/2701) and [MSC2702](https://github.com/matrix-org/matrix-spec-proposals/pull/2702). ([#1935](https://github.com/matrix-org/matrix-spec/issues/1935))