According to https://www.kamailio.org/docs/modules/5.6.x/modules/corex.html#corex.f.file_... the functions cores.file_read() and cores.file_write() do not return a value (return void). The examples after functions are `if (file_write("z', "z")) {...}`. Since the functions appear in `if ()` they return se value.
* document the return values of both functions