How to select an arbitrary MP3 as a ringtone in LineageOS?

Since I do not have access to internal storage in LineageOS and other file browsers, but only to my SD card (or the SD card is commonly referred to as internal storage that would be quite ambiguous), I could put the MP3 file inside Ringtones on the SD card and select the ringtone after a restart.

I suggested replacing this with an intuitive import function in the ringtone selection menu at https://jira.lineageos.org/browse/BUGBASH-1945.