snap - how to grant /tmp access to snap chromium? [duplicate]
I also find this quite annoying. The solution offered here: https://bugs.launchpad.net/ubuntu/+source/chromium-browser/+bug/1851250 is to mount a tmpfs under your home directory.
There is more information here: Why can't Chromium suddenly access any partition except for /home?, along with some other solutions such as installing a non-snap package if you're interested.
I've run into exactly that problem.
It took me some time to understand what was going on as the path /tmp appears in the save as... window (instead of the real path /tmp/snap.chromium/tmp).