Chrome does not fire insertFromDrop beforeinput events for files

[ianstormtaylor]

Chrome does not currently fire insertFromDrop beforeinput events when the drop contains files. (It does fire them if the drop contains HTML or other content instead.)

The bug report is here (please star!):

• https://bugs.chromium.org/p/chromium/issues/detail?id=1028668

This makes it harder to capture all of the drop events properly, since you have to special-case when users specifically drop a file (eg. an image).