Custom ringtones on Nexus 5

I just got my new Nexus 5 through the Google Play Store.  In general it's a great phone!  My only complaint is that it has crashed on me twice -- once during the initial Google Play apps-sync, and again while using the USB 'Media' link to copy several GB  of MP3s onto the phone.

Some of those MP3s are custom ringtones that I'm accustomed to using for particular contacts, i.e., so that I can tell who is calling by the ringtone.  Unfortunately, I could not find any way to select a custom ringtone using any of the built-in menus or apps.  (In the past I've been able to use the built-in music player to set an MP3 as a ringtone but Google Play Music doesn't seem to have that.)

Ultimately what I did was install another file explorer (in this case "ES File Explorer" but I suspect any would work).  Now, when I go into the ringtone selector -- in either Settings > Sound or in the individual Contacts' ringtone -- there is a prompt to choose which file explorer.  If I use ES File Explorer I can then choose any MP3.