allow one additional boolean

the function is not complex and is only called once.
refactoring that seems to be a bit too much effort
and at least out of scope of this pr.

src/mimeparser.rs

@@ -877,6 +877,7 @@ impl MimeMessage {
         Ok(self.parts.len() > old_part_count)
     }
 
+    #[allow(clippy::too_many_arguments)]
     async fn do_add_single_file_part(
         &mut self,
         context: &Context,