You're not doing anything wrong, Nick. As I can see here:
https://freeswitch.org/stash/projects/FS/repos/freeswitch/browse/src/mod/languages/mod_v8/src/fsfile.cpp#172

the whole File class is dummy and wasn't implemented yet in neither v1.6 nor v1.8.

From: Nick Giannak III <nick@hkcradio.com>
To: freeswitch-users@lists.freeswitch.org
Greetings.
In Freeswitch 1.6, the file.exist module does not appear to be implemented in V8. The console shows it as "not implemented". This, however, is a function listed in the documentation. Is it implemented for Freeswitch 1.8? Or, alternatively, are we doing something wrong?
Thanks,
Nick