tries eTag and ContentDisposition headers for file name+format
catches offline and server error/url error scenarios
creates a "recent" file copy if successful, and returns that as a fallback in an error scenario, if it exists
This is to alleviate race conditions where the command sent to clear whatevers previously drawn on a redraw isn't acknowledged til after the redraw, immediately clearing whatever was drawn.
There's probably a better way of doing this - but this works for now.
