My Mac is running out of space, Can I keep file attachments only in the cloud?

If you save the attachment as a file yes. But how many attachments do you have? It would take an awful lot to effect disk space.

How big is your drive? You might want to consider freeing up space on your drive by moving personal data to an external HDD or Cloud storage. I just can't imagine attachments being much of a problem unless you've been accumulating them over many years.