Use content-addressible hash for image filenames

[jamietanna]

To make it easier to deduplicate images, as well as programatically determine the output filename for a media file, could we move to using a content-addressible hash?

For instance, this is what my current media endpoint does:

      slug = DigestUtils.sha256Hex(Base64.getEncoder().encode(file.getBytes())).substring(0, 10);

Inspired by Marty's https://martymcgui.re/2017/01/26/spano-a-minimum-viable-micropub-media-endpoint/