[IMP] web_drop_target: Make sure is a Blob object

Make sure that when a File is dropped into a form the event is propagated and the action only takes place when the element dropped is actually a Blob object.

Taken from #1209

Co-Authored-By: George Daramouskas <daramousk@users.noreply.github.com>

web_drop_target/static/src/js/web_drop_target.js

@@ -110,6 +110,9 @@ odoo.define('web_drop_target', function(require) {
         ) {
             var self = this;
             var file = item;
+            if(!file || !(file instanceof Blob)) {
+                return;
+            }
             var reader = new FileReader();
             reader.onloadend = self.proxy(
                 _.partial(self._create_attachment, file, reader, e, res_model, res_id, extra_data)