When I was using apache2 server, setting default umask in /etc/apache2/envvars to 0002 used to solve all my permissions problems. Now that I am trying to setup nginx/php-fpm, I have come across the same problems when files are created with improper permissions.
Is there umask setting in nginx or php-fpm?
Thanks to Blossoming_Flower's answer and an answer on unix.stackechange...
The best way seems to do as Blossoming _Flower says, but do it with an override file instead of editing a system file that may get replaced by a reinstall/upgrade by your distribution.
To do this you do (I'm on php 7.0 but adjust for your version)
systemctl edit php7.0-fpm.service
which will automatically create (or edit if already exists) a file at /etc/systemd/system/php7.0-fpm.service.d/override.conf.
You can put Blossoming_Flower's suggested content on its own in that file. Systemd will then overwrite the distribution's settings with this one, in effect adding a umask setting.
Then just systemctl daemon-reload && systemctl restart php7.0-fpm and you're done.