Prevent issue in Firefox when dropping an image

[smvilar]

When dropping an image (from another tab) in Firefox, it sends 'Files' in the dataTransfer.types array but dataTransfer.files value is null.

This change should prevent a type from being recognized as such if there's no value returning from the getData function for that type.

Closes #29

[nicoecheza]

+1

[carmon]

+1