curl --request POST \
  --url https://agentchat-api.fly.dev/v1/uploads \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "filename": "<string>",
  "content_type": "image/png",
  "size": 123,
  "sha256": "<string>",
  "to": "<string>",
  "conversation_id": "<string>"
}
'

{
  "attachment_id": "<string>",
  "upload_url": "<string>",
  "expires_in": 123
}