[FIX] web_editor: email receiver see file icon

Scenario:

- add a file in mass mailing editor => an generic icon (instead of lower
  version where icon depended on mimetype) is added linking the file

- save and send mail => no icon is shown in email received

The system is targeting `a[href*="/web/content/"][data-mimetype]:empty`
to add real image inside instead of using background-image attribute
which is stripped in sanitized.

With this commit, data-mimetype is not removed when image processing
attributes are removed when a file is inserted.

opw-2474053

closes odoo/odoo#70022

X-original-commit: eb3772508ce518ff3f84724eebbe43fd1e040d77
Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>

addons/web_editor/static/src/js/wysiwyg/widgets/media.js

@@ -451,7 +451,7 @@ var FileWidget = SearchableMediaWidget.extend({
             }
             href += 'unique=' + img.checksum + '&download=true';
             this.$media.attr('href', href);
-            this.$media.addClass('o_image').attr('title', img.name).attr('data-mimetype', img.mimetype);
+            this.$media.addClass('o_image').attr('title', img.name);
         }
 
         this.$media.attr('alt', img.alt || img.description || '');
@@ -464,6 +464,10 @@ var FileWidget = SearchableMediaWidget.extend({
         removeOnImageChangeAttrs.forEach(attr => {
             delete this.media.dataset[attr];
         });
+        // Add mimetype for documents
+        if (!img.image_src) {
+            this.media.dataset.mimetype = img.mimetype;
+        }
         this.media.classList.remove('o_modified_image_to_save');
         this.$media.trigger('image_changed');
         return this.media;