The function file.chmod() does the same as the appropriate unix command chmod, ie. it changes the permissions on files.
Normally, unix admins use the value in octal because that's much easier:
Digit 1 is for the user, digit 2 for the group and digit 3 for others; the bit
values are 4=execute, 2=write, 1=read.
So: 438 (decimal) is equivalent to 666 (octal), which means read and write permissions to everyone.