Allow images to be dragged anywhere on document editor

frontend/utils/getDataTransferFiles.js

@@ -0,0 +1,18 @@
+// @flow
+export default function getDataTransferFiles(event: SyntheticEvent) {
+  let dataTransferItemsList = [];
+  if (event.dataTransfer) {
+    const dt = event.dataTransfer;
+    if (dt.files && dt.files.length) {
+      dataTransferItemsList = dt.files;
+    } else if (dt.items && dt.items.length) {
+      // During the drag even the dataTransfer.files is null
+      // but Chrome implements some drag store, which is accesible via dataTransfer.items
+      dataTransferItemsList = dt.items;
+    }
+  } else if (event.target && event.target.files) {
+    dataTransferItemsList = event.target.files;
+  }
+  // Convert from DataTransferItemsList to the native Array
+  return Array.prototype.slice.call(dataTransferItemsList);
+}