MuseScore 2.2.1 can't access "/mnt" on Ubuntu 16.04

I'm sure there's some ridiculously simple solution for this, but I don't know what it is. (I am using Ubuntu 16.04 LTS.)
Basically, I've used the MuseScore Portable AppImage for awhile now (think it was v2.1.0), and have been very happy with it. Recently, I decided it was time for a newer version, and found that 2.2.1 was available on the Ubuntu Software Center. It installed with no problems, and works fine for the most part. However, the problem is that MuseScore 2.2.1 installed with the Ubuntu Software Center, cannot access anything inside of "/mnt"--which is where all of my music library is located. If I move files to some other folder on the Linux system, MuseScore 2.2.1 will open them just fine. If I browse to "/mnt" with the Open dialog, it shows nothing inside. If I click on a .MSCZ file in my library (in "/mnt"), MuseScore opens with an error, "File not found." I have not had this problem with any other program on my Ubuntu 16.04 installation, nor did I have this problem when using the AppImage.

Version details: Ubuntu Core 16, Arch.: x86_64, MuseScore version (64-bit): 2.2.1, revision: 51b8386