[FIX] mail: File deletion on module uninstallation

PR #30798 introduced thread, message, and attachments deletion on a
module uninstallation. When removing attachments it was also removing
files in the filestore even if those files were still in use by other
attachments.

opw-1968117

closes odoo/odoo#32682

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>

addons/mail/models/ir_model.py

@@ -25,13 +25,23 @@ class IrModel(models.Model):
         query = "DELETE FROM mail_message WHERE model in %s"
         self.env.cr.execute(query, [models])
 
+        # Get files attached solely by the models
         query = """
-            DELETE FROM ir_attachment
-            WHERE res_model in %s
-            RETURNING store_fname
+            SELECT DISTINCT store_fname
+            FROM ir_attachment
+            WHERE res_model IN %s
+            EXCEPT
+            SELECT store_fname
+            FROM ir_attachment
+            WHERE res_model not IN %s;
         """
+        self.env.cr.execute(query, [models, models])
+        fnames = self.env.cr.fetchall()
+
+        query = """DELETE FROM ir_attachment WHERE res_model in %s"""
         self.env.cr.execute(query, [models])
-        for (fname,) in self.env.cr.fetchall():
+
+        for (fname,) in fnames:
             self.env['ir.attachment']._file_delete(fname)
 
         return super(IrModel, self).unlink()