When you are working on any drag and drop type of application, you may find that the files is empty when you check event.dataTransfer in the Google chrome console on drop event.
Most likely you are getting en empty array due to a bug of Google console. Instead of logging event.dataTransfer to the console and expanding it to see the files array, you can directly log the files array to the console by using event.dataTransfer.files which gives you the length of the array as well of the files list.
Alternatively, you can use Mozilla Firefox console which shows the files list correctly even when you log only event.dataTransfer.
If you are getting the error due to any other reason, let us know through a comment below.