How can I determine if the mildew on the bathroom ceiling comes from the upper floor?
Is the mold concentrated is a small area or cover most of the ceiling? Do you notice water condensation on any surfaces after using the shower? If the morror fogs or the tiles feel wet  your shower is the most likely cause. If it were from a leak upstairs you should see ceiling discoloration in an area ajacent to the leak. If your bathroom has a vent fan or window use it. Leave the door open to allow air to circulate after showering. If you can paint the ceiling with bathroom specific paint as it contains a mildewcide. The fix is to control the moisture so that the mold can't start.