Activity form "type-based extension" feature doesn't support custom paths

There's some code near the bottom of CRM_Activity_Form_Activity::preprocess() that checks if a file with the same name as the activity type exists under a subfolder called Activity, and if so, it gets inserted into the activity form. I believe this was originally put in for Case but was made general enough to work for all activity types. However, in CRM_Activity_BAO_Activity::getFileForActivityTypeId(), it does a check for the file's existence but only looks under $civicrm_root.

It should also look under the custom php path, if set.