file permission issue -- file attachment in mass mail
It's seems like a bug to me. I can reproduced it on at least one of my site.
The permission used to be 660 and is now 600 with the new angular CiviMail.

Can you log an issue in JIRA ?

In the mean time, I think it's possible to use legacy UI instead of Angular but can't find how :(

EDIT

In my case, i am able to make it work when running the task manually because it's then run as www-data. But it doesn't work when the process is run by cron.