Attachments

The Linq API supports sending rich media files including images, videos, documents, audio, and contact cards. There are two ways to include media in your messages. For the basics of sending messages, see Sending Messages. For iMessage voice memos with inline audio playback, see Voice Memos.

Direct URL (up to 10MB)

Provide a publicly accessible HTTPS URL directly in a media message part. This is the simplest approach for most use cases:

curl -X POST https://api.linqapp.com/api/partner/v3/chats/{chat_id}/messages \
  -H "Authorization: Bearer $LINQ_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "message": {
      "parts": [
        { "type": "text", "value": "Check out this photo!" },
        {
          "type": "media",
          "url": "https://example.com/photo.jpg"
        }
      ]
    }
  }'

await client.chats.messages.send(chatId, {
  message: {
    parts: [
      { type: 'text', value: 'Check out this photo!' },
      { type: 'media', url: 'https://example.com/photo.jpg' },
    ],
  },
});

client.chats.messages.send(
    chat_id,
    message={
        "parts": [
            {"type": "text", "value": "Check out this photo!"},
            {"type": "media", "url": "https://example.com/photo.jpg"},
        ],
    },
)

Note: MIME type is inferred server-side from the file. You do not need to pass a mime_type on media parts.

Note: The URL must be publicly accessible via HTTPS with a valid SSL certificate. The API downloads the file from the URL before sending.

Pre-upload (up to 100MB)

For larger files or files you send repeatedly, use the three-step pre-upload flow:

Step 1: Request a pre-signed upload URL

curl -X POST https://api.linqapp.com/api/partner/v3/attachments \
  -H "Authorization: Bearer $LINQ_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "filename": "photo.jpg",
    "content_type": "image/jpeg",
    "size_bytes": 245760
  }'

const attachment = await client.attachments.create({
  filename: 'photo.jpg',
  content_type: 'image/jpeg',
  size_bytes: 245760,
});

attachment = client.attachments.create(
    filename="photo.jpg",
    content_type="image/jpeg",
    size_bytes=245760,
)

filename, content_type, and size_bytes are all required. The response includes:

• attachment_id — permanent ID to reference in media parts (never expires)
• upload_url — presigned URL to PUT the file to (expires in 15 minutes)
• download_url — permanent CDN URL for the uploaded file
• http_method — always PUT
• expires_at — when upload_url expires
• required_headers — headers you must send on the upload request exactly as returned (S3 rejects the upload otherwise)

See the Create Attachment endpoint for the full response.

Step 2: Upload the file

PUT the raw file bytes to upload_url. You must include every header from required_headers exactly as returned — the URL is signed against those values:

curl -X PUT "${upload_url}" \
  -H "Content-Type: image/jpeg" \
  --data-binary @photo.jpg

The body is the binary file content — not JSON, not multipart form data.

Step 3: Send the message with the attachment

Reference the attachment_id in your message:

curl -X POST https://api.linqapp.com/api/partner/v3/chats/{chat_id}/messages \
  -H "Authorization: Bearer $LINQ_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "message": {
      "parts": [
        {
          "type": "media",
          "attachment_id": "a1b2c3d4-e5f6-7890-abcd-ef1234567890"
        }
      ]
    }
  }'

Tip: Attachment IDs never expire — reuse them across multiple messages without re-uploading. This is ideal for logos, templates, or frequently sent media.

Supported file types

Category	Formats
Images	JPEG, PNG, GIF, HEIC, HEIF, TIFF, BMP
Videos	MP4, MOV, M4V, MPEG, MPG, 3GP
Audio	M4A, AAC, MP3, WAV, AIFF, CAF, AMR
Documents	PDF, TXT, RTF, CSV, ZIP, EPUB, HTML, HTM, Microsoft Office (DOC, DOCX, XLS, XLSX, PPT, PPTX), Apple iWork (PAGES, NUMBERS, KEY)
Contacts & Calendar	VCF, ICS

Not supported: WebP, SVG, FLAC, OGG, and executable files are explicitly rejected.

File size limits

Method	Max size
Direct URL	10 MB
Pre-upload	100 MB

CDN domain

Uploaded files and media in webhook payloads are served from cdn.linqapp.com. Add this domain to your allowlist if you have network restrictions.