Support webp in _get_mime_type_for_image

untested

discord/utils.py

@@ -252,6 +252,8 @@ def _get_mime_type_for_image(data):
         return 'image/jpeg'
     elif data.startswith(b'\x47\x49\x46\x38\x37\x61') or data.startswith(b'\x47\x49\x46\x38\x39\x61'):
         return 'image/gif'
+    elif data.startswith(b'RIFF') and data[8:12] == b'WEBP':
+        return 'image/webp'
     else:
         raise InvalidArgument('Unsupported image type given')