Ability to scan a file before it's uploaded.

**Is your feature request related to a problem? Please describe.**

Some users would prefer to have their client scan on upload rather than just download, to prevent problematic files appearing on the server in the first place. This seems reasonable enough.

**Describe the solution you'd like**

An endpoint to scan a body of a file (optionally with an encrypted component) which performs the same check as the download routes, without having to use the content repository.

**Describe alternatives you've considered**

A transparent proxy to the CS /upload endpoint was considered but rejected. Clients could bypass the scanning with an encrypted file, so this needs to be done as two steps. One step for the client to test their file on the content scanner, and then once successful to upload the file to the homeserver. Scanning on download will remain in place for other clients so the upload scan merely serves as a friendly protection to the uploader.

**Additional context**

I have a PR for this in waiting, I do not expect any development other than review :)

Beginne mit der Prüfung des im Issue erwähnten wartenden PR, da die angeforderte Arbeit bereits im Gange ist. Überprüfe, dass er einen Endpoint zum Scannen des Inhalts einer Datei hinzufügt, optional mit einer verschlüsselten Komponente, wobei dieselbe Prüfung wie bei Download-Routen ohne das Content-Repository verwendet wird; das vorhandene Scannen von Downloads sollte erhalten bleiben.

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