duration_secs? | float | the duration of the audio file (currently for voice messages)
waveform? | string | base64 encoded bytearray representing a sampled waveform (currently for voice messages)
flags? | integer | attachment flags combined as a bitfield
https://github.com/discord-php/DiscordPHP/blob/a8a617dcc6f55e15dc2ec43d97d7836d3fbaedca/src/Discord/Parts/Channel/Attachment.php#L16-L33
See https://discord.com/developers/docs/resources/channel#attachment-object-attachment-structure