[FIX] web: Email attachment issue

Steps to reproduce the bug:

- Go on a SO and click on "Send by email"
- Remove the attachment and click on "Add an attachment"
- Choose an attachment A and remove it by clicking on the cross
- Click on "Add an attachment" and choose the attachment A

Bug:

The attachment didn't appear in the modal.

opw:1862482

addons/web/static/src/js/fields/relational_fields.js

@@ -1563,6 +1563,7 @@ var FieldMany2ManyBinaryMultiFiles = AbstractField.extend({
 
         this.$('form.o_form_binary_form').submit();
         this.$('.oe_fileupload').hide();
+        ev.target.value = "";
     },
     /**
      * @private