[FIX] base: keep mimetypes after force_storage

The write override recomputed the mimetype when changing the datas
It may give a different result

closes odoo/odoo#24397

Signed-off-by: Martin Trigaux (mat) <mat@odoo.com>

odoo/addons/base/models/ir_attachment.py

@@ -65,7 +65,8 @@ class IrAttachment(models.Model):
         }[self._storage()]
 
         for attach in self.search(domain):
-            attach.write({'datas': attach.datas})
+            # pass mimetype, to avoid recomputation
+            attach.write({'datas': attach.datas, 'mimetype': attach.mimetype})
         return True
 
     @api.model