Thanks @Manish !
A simple trick that worked for me (Android 8) : if you upload the audio file right in the special folder “Ringtones”, just aside “Download”, “Documents”, “DCIM” … etc (under /storage/emulated/0), you’ll see it appear automatically in the Phone ringtone pick list, at step 5 (without need to “Add ringtone”).
And it works the same for “Alarms” and “Notifications” 