Currently when saving field data config context is being used. There may be some reasons for doing that, but this just indicates that something is missing in Moodle core APIs that pushes developers to workaround them and this should be fixed in core.
So rough tasks here would be:
figure out core API limitations that forced to use config context
if there are any, then raise MDL to fix those limitations
when MDL is fixed (or if not needed), implement context using instance context instead of config context
This came out as part of discussion in https://github.com/andrewhancox/moodle-customfield_file/issues/9
Currently when saving field data config context is being used. There may be some reasons for doing that, but this just indicates that something is missing in Moodle core APIs that pushes developers to workaround them and this should be fixed in core.
So rough tasks here would be:
@andrewhancox does this sound ok to you?