I used two of my IMAP-based email accounts as a test and ended up with a positive result in Mail running under MacOS 10.11.5. In the email account that I used to receive a 49MB file, I went to Mail >Preferences > Accounts and selected the account I wanted to use as the receiving account. Once that selection was made, I clicked the Advanced button on the right hand side of the window and, then, unchecked Automatically download all attachments. I closed the Preferences window and saved the changes. I then used another IMAP account to send the account I had just altered the 49MB file. When I opened the the message in the receiving account, it briefly appeared in the small format and, then, in a few seconds, it appeared in its larger 49MB size. When I checked its download size from the dropdown, it was, indeed, 49MB. I ran this test three times with three different images and it worked in each case. Granted this is a workaround and the bug needs to be resolved, but at least it works!