When I run a config that creates a thumbnail of an MP4 video, and have it store to S3, the resulting JPG file gets the MPEG-4 content type. When trying to load the URL of the image directly in the browser, it tries to download it instead of display it. It displays fine if you use <img src="[url]"> but it should still be the right content type.

The content type needs to be either determined from the file created or hard-coded with the jpg/png/gif content types and sent along with the file being uploaded.