Return 404 NotFound if requested attachment does not exist (#20886)
Add code to test if GetAttachmentByID returns an ErrAttachmentNotExist error
and return NotFound instead of InternalServerError
Fix #20884
Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
zeripath