Basically, you need to clear the system default ringtone file picker...(step 3)

Here are the steps to choose a custom .MP3 ringtone once it's on your Android M/Nexus 5X:

1.  install a 3rd party file browser such as Solid Explorer from the Google Play Store
2.  from Settings/Apps, click on the 3 dots upper right corner and click 'Show system'
3.  from Media Storage/Open by default, click 'CLEAR DEFAULTS'
4.  from Settings/Sound & notification, click 'Phone ringtone', 'Complete action using' Solid Explorer (or your preferred file browser app)
5.  navigate to the folder where you saved your custom ringtone and select the appropriate .MP3 file

That's it.