I shot a 3-second Motion Timelapse on my Android phone. It plays great on the phone, shows up in the Gallery. No problem there. But, when I try to copy the MP4 file to a thumb drive or Dropbox, the file displays all solid black for the 3 seconds. What am I doing wrong?

I shot a 3-second Motion Timelapse on my Android phone. It plays great on the phone, shows up in the Gallery. No problem there. But, when I try to copy the MP4 file to a thumb drive or Dropbox, the file displays all solid black for the 3 seconds. What am I doing wrong?