Not sure if you got a solution for this. I was trying a workflow which required to check if there is an attachment available in the item and was able to work it out. Seems MS has rolled out something which was not present earlier. There is a dynamic content called "Has attachments" now which checks if there is an attachment available, the expression would work as follows
@equals(body('Update_item')?['{HasAttachments}'], bool('false'))
The Update_item was a previous action I called, so it could be any action which can call the item.
Regards,
Leejoh