Attachments: Fixed full range request handling

Attachments: Fixed full range request handling

We were not responsing with a range request, where the requested range
was for the full extent of content. This changes things to always
provide a range request, even for the full range.

Change made since our existing logic could cause problems in chromium
browsers.

Elseif statement removed as its was likley redundant based upon other
existing checks.
This also changes responses for requested ranges beyond content, but I
think that's technically correct looking at the spec (416 are for when
there are no overlapping request/response ranges at all).

Updated tests to cover.
For #5342