I am having a similar experience. I do not know yet the method below will download attachments.
Within Trello, click the round button in the top right corner of your screen, inside of which will be your account name initials.
Go to settings.
Scroll down to "Personal Data Export"
Click the button: Download Personal Data
You should have a new tab open with multiple lines of code. Right click anywhere over the screen and "Save As" to export that data to a location of your choice.
You can track my thread regarding importing backed up data here:
One way to back-up data is to Copy Board, change the board name to something like 20200727_boardname_backup, then close(archive) that board.
It won't survive Trello imploding, but it'll save you when you accidentally delete anything or have a Butler rule go wild etc.
I believe it copies everything except comments and activity logs, though you'll probably want to double check to be sure.
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