There is no way to edit an attachment message block without text.
A message with an attachment (without text) can be edited by adding text and changing it through subsequent editing.
A message with an attachment (without text) does not have an “edit” function, and you must re-send the file and write the text.
yip, that’d be very useful.
also in combination with “sending multiple images”, you suddenly have a way to send 10 holiday photos in a batch and comment afterwards, “this is me at the beach”, “this is me eating” 
to be clear: this issue is only about raw attachments without text where we skipped “edit” to simplify code paths that time; editing text of attachments with text is already possible