Improve mimetype detection for object storages

Object storage instances always fall back to the content based mimetype detection, because the file name for object storage was always random due to the fact that it was temporarily storage in a generated temp file. This patch adds a check before that to make sure to use the original file name for this purpose and also remove possible other extensions like the versioning or part file extension.

Signed-off-by: Morris Jobke <hey@morrisjobke.de>