dealing with invalid drag and drops

frontend/src/upload.js

@@ -143,6 +143,13 @@ $(document).ready(function() {
 
     let file = '';
     if (event.type === 'drop') {
+      if (!event.originalEvent.dataTransfer.files[0]) {
+        $('.upload-window').removeClass('ondrag');
+        document.l10n.formatValue('uploadValidFile').then(str => {
+          alert(str);
+        });
+        return;
+      }
       if (
         event.originalEvent.dataTransfer.files.length > 1 ||
         event.originalEvent.dataTransfer.files[0].size === 0

public/locales/send.en-US.ftl

@@ -11,6 +11,7 @@ uploadPageSizeMessage = For the most reliable operation, it’s best to keep you
 uploadPageBrowseButton = Select a file on your computer
   .title = {uploadPageBrowseButtonTitle}
 uploadPageMultipleFilesAlert = Uploading multiple files or a folder is currently not supported.
+uploadValidFile = Please drag a valid file to upload.
 
 uploadPageBrowseButtonTitle = Upload file