To fix this issue Since I neither have the drop box app source or access to the actual file here is my best case guess that I will make on basis of a couple of assumptions. The file is not playable by the native app because it is in a format that cannot be played there. Can you say download the file, put it in a sd card and then play it? If not, then the phone does not support this format.
Android media player\video issue (First video get frozen when i play 2nd)