Dropping files in voxel-drop is broken in Chrome 48, works in Firefox

[deathcap]

Regressed sometime in Chrome - I get the drop event, but no data is read

See analysis in voxel/voxelmetaverse#77

[deathcap]

https://github.com/voxel/voxel-artpacks (default keybinding: 'P') works in the meantime to change texture packs (interestingly, can drag into there and it works fine)