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
Until it’s possible to add a text to attachments a “quick and dirty” workaround would be to add a space to every attachment you send. Nobody will see it and it looks like an attachment without text but you are still able to edit it and add text afterwards 
Funny side fact (and I think this could be a bug): Adding a space is only possible if you are posting the attachment for the first time 
You are not allowed to keep the space as text if you are editing or you get this error message (tested on the just-released DC Desktop v2.48.0):
EDIT:
The workaround doesn’t work with ArcaneChat. Don’t know about DC Android as I don’t use this client.
