When a Content exists with a BinaryFile of mimetype a/something, and it is updated to a new file of mimetype b/something, PlatformUI will send the currently mimeType (a), and the REST API will use this value to store the file. The result is that the file can't be downloaded with the download controller (not found).

The reason is that it uses the mimeType that is passed instead of building its own.

On one hand, PlatformUI should maybe send a correct value, but otoh, it isn't required. Passing a wrong mimeType as input shouldn't mess up the stored data, as it is not supposed to have any kind of effect.