Ah, I see. I don't think that I provided enough detail in my original post. The problem is not filedir, the whole moodledata structure including filedir, trashdir, cache, etc., is in a different directory under /var/www and its size has not changed significantly.
The directory where I am seeing the problem is /home/moodle/autocoursebackups which is the location that I specified under: Site administration > Courses > Backups > Automated backup setup, backup_auto_destination. I have automated backups running four times per week so the directory is normally populated with files that are named in a manner similar to : "backup-moodle2-course-12-japan134su16-20190331-0503-nu.mbz".
In the sixteen days since I updated from '3.6.1+ (Build: 20181220)' to '3.6.3+ (Build: 20190322)' there have been new files added to the autocoursebackups directory that are named similarly to "backup-moodle2-activity-261-261-20190409-0414.mbz". Could these be files that should have been saved in filedir or some other folder under moodledata but wound up here by mistake?
Natassia