Somewhere in system settings, their names and locations are different in different Linux distributions.
You could try direct editing of /etc/samba/smb.conf (root permissions are required). There you find a section named like your share name (e.g. if in Windows you disk is mapped to machine_nameMyShare, then you look for the section [MyShare] in smb.conf). Within this section, if there is create mask option already, just modify its value to 0644. Otherwise append a new line with this option:
create mask = 0644
(space within “create mask” is not a typo!). Save the file and restart the Samba service. The way to do it, again, depends on your Linux system, I’d try the command
(again, with root permissions), or simply reboot the machine.