Provisional 'open file' command on detail page

Opens a SEND intent with EXTRA_STREAM of URI to the original image.
Unfortunately most apps don't seem to grok an HTTPS URI here, they
expect either content: or file:.

Will probably have to download the file locally, then push out with
a file: URI.
refactor
Use DownloadManager to download the file to public downloads dir.
Add icons! stock 'upload' icon, flipped for 'download' icon