Handle cases where DataTransfer has a null +files+ slot

src/DOM/HTML/Event/DragEvent/DataTransfer.js

@@ -1,6 +1,6 @@
 "use strict";
 
-exports.files = function (dataTransfer) {
+exports.filesNullable = function (dataTransfer) {
   return dataTransfer.files;
 };
 

src/DOM/HTML/Event/DragEvent/DataTransfer.purs

@@ -1,16 +1,26 @@
 module DOM.HTML.Event.DragEvent.DataTransfer
   ( DataTransfer
   , files
+  , filesNullable
   , types
   ) where
 
+import Prelude
 import DOM.File.Types (FileList)
+import Data.Maybe (Maybe)
+import Data.Nullable (Nullable, toMaybe)
 
 foreign import data DataTransfer :: *
 
 -- | Contains a list of all the local files available on the data transfer.
 -- | Empty if the drag operation doesn't involve dragging files.
-foreign import files :: DataTransfer -> FileList
+-- |
+-- | It's possible that a drag operation may have null files, instead of an
+-- | empty file list. In these cases Nothing is returned.
+files :: DataTransfer -> Maybe FileList
+files = toMaybe <$> filesNullable
+
+foreign import filesNullable :: DataTransfer -> Nullable FileList
 
 -- | Returns an array of data formats used in the drag operation.
 -- | If the drag operation included no data, then the array is empty.