Why must you not hold a door open with a fire extinguisher?
If it's a fire or smoke door, it must be able to close automatically to carry out its design function during a fire. If the door is propped open with ANYTHING, the fire and smoke may spread uncontrollably and cause severe injury and fire spread.
Also, it is a code violation to set a fire extinguisher on the floor, unless it is on wheels. Also, it is a code violation to place any obstacle within the clear width of any escape doorway. Also, if the fire extinguisher is being used to prop open a door, then it is not where it should be when it may be needed. The proper location may have been designed specifically to fulfill a distance requirement.
If caught by a fire inspector, this single act could result in four or more violations.