[FIX] mail: add missing checksum convert data

Follow up of send is_main with the attachments.

task-2783076

closes odoo/odoo#87795

X-original-commit: 2eafb395afb0262f2b789e377932ac191db1fdcc
Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>
Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>

addons/mail/static/src/models/attachment.js

@@ -15,6 +15,9 @@ registerModel({
          */
         convertData(data) {
             const data2 = {};
+            if ('checksum' in data) {
+                data2.checksum = data.checksum;
+            }
             if ('filename' in data) {
                 data2.filename = data.filename;
             }