UOP is the file extension used in EC... MUL is 2d, unless that's been changed.
With SA we updated the legacy animation system to handle two animation formats. The older more restricted format that is packed into mul files, and the newer EC animation format which is much more flexible and packed into uops.
All animation files pre SA expansion and any human/elf wearables use the old system.
New mobs and gargoyle assets use the uop system.
That's why you see animation files in both formats.