MF doesn’t work on Special Chests and Sparkling Chests. You may distinguish them from normal ones by great amount of items they may drop in full game.
Special Chests are normal looking ones. For example go back to Blood Moor from Cold Plains WP. Frequently you may found a house with a chest near the exit from the Moor. It is Special Chest.
Special Chests have fixed chances for item qualities. 5% of their drops are rare items, 95% — magic. So it is impossible to find unique/set/high quality/low quality items from them. MF isn’t working.
Sparkling Chests looks sparklingly.

However IFAIK not all shiny chests are “Sparkling†for some unknown reason. For example Mausoleum/The Pit/Ancient Tunnels shiny chests are Sparkling.
Sparkling Chests have fixed chances too:
2% — unique
4% — set
6% — rare
**% — magic
Also these chances are for whole chest drop, so if the game decides to drop unique items, all items in the chest will be unique. In most cases it will be just one item, though.
MF works normally for all other “Objects†— “Normal†chests, rocks, corpses, etc.