support storing real content type

[guruofgentoo]

For S3, all objects uploaded via a signed link get the octet-stream content type. That's stored in S3 along with the object as metadata, and gets passed back when the object is requested.

In most cases, that works just fine. But: when an image from S3 is on a page, and the user opens the image in a new tab, the browser will look at that content type and automatically download the file (rather than displaying the image in a tab).

As of 0.5.11, we can inject a content type when getting a download link to workaround this issue. But, we really should allow an app to provide a correct content type at upload time.