refactor: remove some easy to remove unwrap() calls

src/mimeparser.rs

@@ -679,8 +679,9 @@ impl MimeMessage {
     fn parse_system_message_headers(&mut self, context: &Context) {
         if self.get_header(HeaderDef::AutocryptSetupMessage).is_some() && !self.incoming {
             self.parts.retain(|part| {
-                part.mimetype.is_none()
-                    || part.mimetype.as_ref().unwrap().as_ref() == MIME_AC_SETUP_FILE
+                part.mimetype
+                    .as_ref()
+                    .is_none_or(|mimetype| mimetype.as_ref() == MIME_AC_SETUP_FILE)
             });
 
             if self.parts.len() == 1 {