`unstable_parseMultipartFormData`

Allows you to handle multipart forms (file uploads) for your app.

Would be useful to understand the Browser File API to know how to use this API.

It's to be used in place of request.formData().

- const formData = await request.formData();
+ const formData = await unstable_parseMultipartFormData(request, uploadHandler);

For example:

export const action = async ({ request }: ActionArgs) => {
  const formData = await unstable_parseMultipartFormData(
    request,
    uploadHandler // <-- we'll look at this deeper next
  );

  // the returned value for the file field is whatever our uploadHandler returns.
  // Let's imagine we're uploading the avatar to s3,
  // so our uploadHandler returns the URL.
  const avatarUrl = formData.get("avatar");

  // update the currently logged in user's avatar in our database
  await updateUserAvatar(request, avatarUrl);

  // success! Redirect to account page
  return redirect("/account");
};

export default function AvatarUploadRoute() {
  return (
    <Form method="post" encType="multipart/form-data">
      <label htmlFor="avatar-input">Avatar</label>
      <input id="avatar-input" type="file" name="avatar" />
      <button>Upload</button>
    </Form>
  );
}

`uploadHandler`

The uploadHandler is the key to the whole thing. It's responsible for what happens to the multipart/form-data parts as they are being streamed from the client. You can save it to disk, store it in memory, or act as a proxy to send it somewhere else (like a file storage provider).

Remix has two utilities to create uploadHandlers for you:

unstable_createFileUploadHandler
unstable_createMemoryUploadHandler

These are fully featured utilities for handling fairly simple use cases. It's not recommended to load anything but quite small files into memory. Saving files to disk is a reasonable solution for many use cases. But if you want to upload the file to a file hosting provider, then you'll need to write your own.