Cannot send an empty message 50006 when using multipart Laravel Http post - laravel

Question

I am getting this error when trying to send a message to a Discord webhook.
{"message": "Cannot send an empty message", "code": 50006}
I've read the discord documentation on sending images, and can honestly not see what I'm doing wrong.
Here's the code I'm running:
Http::withHeaders([
'Content-Type' => 'multipart/form-data; boundary=--abcdefghijklmnop'
])
->asMultipart()
->post(
$this->webhookUrl,
[
[
'name' => 'payload_json',
'contents' => '{
"content": "Hello, World!",
"embeds": [{
"title": "Hello, Embed!",
"description": "This is an embedded message.",
"thumbnail": {
"url": "attachment://image1.jpg"
},
"image": {
"url": "attachment://image2.jpg"
}
}],
"attachments": [{
"id": 0,
"description": "Image of a cute little cat",
"filename": "image1.jpg"
}, {
"id": 1,
"description": "Rickroll gif",
"filename": "image2.jpg"
}]
}',
'headers' => [
'Content-Type' => 'application/json',
]
],
[
'name' => 'image1.jpg',
'contents' => 'data:image/jpeg;base64,[[encoded image]]',
'headers' => [
'Content-Type' => 'image/jpeg',
]
],
[
'name' => 'image2.jpg',
'contents' => 'data:image/jpeg;base64,[[encoded image]]',
'headers' => [
'Content-Type' => 'image/jpeg',
]
]
];
);
As you can see, this is basically the message they use in their documentation as an example and I've just translated it to be sent with the Http facade.
What am I missing?