Will eventually replace file-control.php. Carries out same file checks
etc, plus defines error info, form looparound system gone in place of
2nd XHR call on save as/new file save. All JS moved into a doNext string
to be run on success (no errors).
The only output here is the JSON response once we have carried out our
processes.
Note: Diff loading/MDT mismatch system not done yet.
