I had a network disconnection right after clicking on one of the sample pictures.
That caused the initiated download to be corrupt/empty. AD created an empty file with 0 byte and tried to open it.
This causes an error: "The file type is not supported."
Of course you can reproduce it: right click on in an Explorer-Window --> new --> Text file --> name the file "emptyfile.afdesign".
Then try to open it.
So I see 2 cases to handle:
Check downloads were completed
handle empty files with extension .afdesign