Don't lie about the preview mimetype

For legacy reasons we stored all the previews with a png extention.
However we did not put png data in them all the time.

This caused the preview endpoints to always report that a preview is a
png file. Which was a lie.

Since we abstract away from the storage etc in the previewmanager. There
is no need anymore to store them as .png files and instead we can use
the actual file extention.

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>