Better support for HEIC files

[PoeHaH]

First of all: thank you for maintaining this! Not sure what happened to the original Dropzone but it's widely used so I'm happy to see interest in further keeping this alive :-).

Images taken with iPhone cameras are often stored in a raw format with ".heic" extension. Their mime type is "image/heic" so Dropzone tries to render a thumbnail preview, but it doesn't work:

In the source code, we can see why the thumbnail can't be displayed:

In short, the dataUrl is not a string, but an object.

I propose to fix this by making a simple change to the thumbnail() function to check if the dataUrl is a string.

Would you be open to add this into the next release?