I downloaded an audio file (a podcast, 41MB, .mp3). After it downloaded, I tried playing the file by selecting the file via the pull-down menu. I also tried by opening up the Downloads app and selecting the file (not long-pressing). Both of these methods opened the .mp3 file in the c:geo Geocaching app, which treated my action as Importing a Geocache map file (Geocaching is a GPS-based scavenger hunt or letter-box game). c:geo opened up its List feature, which is used for storing and sorting Lists of Geocaches, which you would usually store by Location.
I tried to prevent this behavior by looking in the Google Music, Downloads and c:geo application Defaults, to determine whether there was something I could do there (i.e. clear some setting that allowed c:geo to open .mp3 files by default). This may be part of the solution, but I did not succeed.
Does anyone know a straight-forward method to open and play downloaded .mp3 files? The ideal solution would utilize Google Play Music, but that's not a requirement; not deleting the c:geo app would also be preferred.
The device is an LG Nexus 5, but I'm not tagging the question with that info since I doubt it is device-specific.