Attachments and uploader

FormThunder accepts files in two ways, both on Pro and Agency, and only on forms with attachments turned on. A direct multipart upload handles files up to 5 MB each, in a request up to 6 MB. The advanced uploader sends files to storage first, which allows up to 25 MB per file. Files are attached to the email and linked from the dashboard.

Turn attachments on

Set attachments_enabled on the form in the dashboard, or through your AI, the CLI or the API. Otherwise, files are ignored and the response carries a warning. The rest of the submission still arrives.

Direct upload

<form action="https://formthunder.com/submit" method="POST" enctype="multipart/form-data">
  <input type="hidden" name="access_key" value="YOUR_ACCESS_KEY">
  <input type="email" name="email" required>
  <input type="file" name="attachment" required>
  <button type="submit">Send</button>
</form>

With fetch, pass a FormData object and leave out the Content-Type header, so the browser can set the boundary.

await fetch("https://formthunder.com/submit", { method: "POST", body: new FormData(form) });

Advanced uploader

The helper script turns a file input into a multi-file uploader.

<input type="file" name="attachment"
  data-advanced="true"
  data-max-files="5"
  data-max-file-size="25mb"
  accept=".pdf,.png,.jpg">

<script src="https://formthunder.com/client/script.js" async></script>

Uploading from your own code

Ask for a signed URL, PUT the file there, then submit its ID in the attachment field.

const sign = await fetch("https://formthunder.com/api/uploads/sign", {
  method: "POST",
  headers: { "Content-Type": "application/json" },
  body: JSON.stringify({ access_key: key, filename: file.name, size: file.size, type: file.type }),
}).then((r) => r.json());

await fetch(sign.url, { method: sign.method, headers: sign.headers, body: file });

await fetch("https://formthunder.com/submit", {
  method: "POST",
  headers: { "Content-Type": "application/json" },
  body: JSON.stringify({ access_key: key, email, attachment: [sign.upload_id] }),
});

How files reach you

Files are attached to the notification email up to a total size. Past that, the email links to them instead. Webhook payloads list each file's name, size, type and a download URL.

Treat files from a public form with the same care as any email attachment.