We now detect whether a file is being dropped by a user (by checking `event.dataTransfer.files.length`) and prevent the remaining drop behavior if this is the case. Otherwise, drop happens like normal (so a user can still drop text into the editor, or even an image that has been uploaded already via normal means and rendered in the editor).
Previously, when editing a post with an unordered list in it via the
Classic editor, backslashes (\) would get added to the end of each list
item. This fixes that.
Closes#480
