Amaze File Manager. Actively developed, material design and GPLv3 licensed. You can find it on the Play Store or F-Droid.
Edit: Do you mean an alternative gallery application? After reading again the comment I don't know if that was the case.
This is what I use and haven't had any issues with root, although if I remember correctly, I had to try a little back and forth to get it to request access through privacy guard.. Because it on F-Droid it means it is open source and fairly popular/active.