This did not work:

Setting the umask of the Apache user

Arch does not possess a /etc/apache2/envvars file, and altering the /etc/rc.d/httpd script did not change anything.

Maybe someone can enlighten me how/where you can exactly alter the rc-script, so apache comes with an umask 002

After trying again, oddly editing the /etc/rc.d/httpd file works.

Just put

umask 0002

Immediately after the #!/bin/bash and everything works because it should.

No clue why it did not work before, but well...