Add support for voice messages

Co-authored-by: Danny <1695103+Rapptz@users.noreply.github.com>

discord/message.py

@@ -183,6 +183,14 @@ class Attachment(Hashable):
         Whether the attachment is ephemeral.
 
         .. versionadded:: 2.0
+    duration: Optional[:class:`float`]
+        The duration of the audio file in seconds. Returns ``None`` if it's not a voice message.
+
+        .. versionadded:: 2.3
+    waveform: Optional[:class:`bytes`]
+        The waveform (amplitudes) of the audio in bytes. Returns ``None`` if it's not a voice message.
+
+        .. versionadded:: 2.3
     """
 
     __slots__ = (
@@ -197,6 +205,8 @@ class Attachment(Hashable):
         'content_type',
         'description',
         'ephemeral',
+        'duration',
+        'waveform',
     )
 
     def __init__(self, *, data: AttachmentPayload, state: ConnectionState):
@@ -211,11 +221,19 @@ class Attachment(Hashable):
         self.content_type: Optional[str] = data.get('content_type')
         self.description: Optional[str] = data.get('description')
         self.ephemeral: bool = data.get('ephemeral', False)
+        self.duration: Optional[float] = data.get('duration_secs')
+
+        waveform = data.get('waveform')
+        self.waveform: Optional[bytes] = utils._base64_to_bytes(waveform) if waveform is not None else None
 
     def is_spoiler(self) -> bool:
         """:class:`bool`: Whether this attachment contains a spoiler."""
         return self.filename.startswith('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
+
     def __repr__(self) -> str:
         return f'<Attachment id={self.id} filename={self.filename!r} url={self.url!r}>'