Fix attachment is_spoiler() and is_voice_message()

discord/message.py

@@ -254,11 +254,12 @@ class Attachment(Hashable):
 
     def is_spoiler(self) -> bool:
         """:class:`bool`: Whether this attachment contains a spoiler."""
-        return self.filename.startswith('SPOILER_')
+        # The flag is technically always present but no harm to check both
+        return self.filename.startswith('SPOILER_') or self.flags.spoiler
 
     def is_voice_message(self) -> bool:
         """:class:`bool`: Whether this attachment is a voice message."""
-        return self.duration is not None and 'voice-message' in self.url
+        return self.duration is not None and self.waveform is not None
 
     def __repr__(self) -> str:
         return f'<Attachment id={self.id} filename={self.filename!r} url={self.url!r}>'