improve mime map with mimemagic gem, OPSK-37

Conflicts:
	Gemfile.lock
OPSK-37 these images have been moved to asset path
OPSK-37: content type of unknown file types will be updated before save, while  non-specific types, e.g. application/octet-stream, are still stored in the database, but refined when reading the attribute
Merge remote-tracking branch 'vln/master' into vln-merging

Conflicts:
	app/models/content_blob.rb
Merge branch 'vln-merging'