Need web service to find contextid to use in core_files_upload

The 'core_files_upload' web service has a 'contextid' parameter but there is no way from web services to find the contextid. You can reasonably expect an external user to find the cmid of (say) a module but the contextid for that will be entirely internal.

This is quite a serious omission and there have been a few threads in the forums moaning about it.