Support FormData in SvelteKit remote function

Describe the problem

Currently, SvelteKit’s remote function works well with JSON payloads but does not provide direct support for sending FormData.
This becomes limiting when dealing with use cases that require file uploads (images, PDFs, etc.) or mixing text fields and binary data in the same request.

• Enable seamless integration with forms that include file uploads.

• Allow remote function to be used consistently for both JSON and FormData.

• Avoid the need to fall back to manual fetch just for handling uploads.

Describe the proposed solution

import {  command } from '$app/server';

export const commandFormData = command(async (data: FormData) => {
    if (!(data instanceof FormData)) {
      throw new Error('Invalid form data')
    }
    const name = data.get('name')
    const age = data.get('age')
    const file = data.get('file')

    if (!name || !age) {
      throw new Error('Name and age are required')
    }

    return {
      name: name.toString(),
      age: parseInt(age.toString(), 10),
      file: file.name
    }
});

<script lang="ts">
	import { commandFormData } from './data.remote';
</script>

<button
	onclick={async () => {
		try {
                const formData = new FormData();
                formData.append('name', 'Name');
                await commandFormData(formData);
		} catch (error) {
			console.error(error);
		}
	}}
>
	Command with Form Data
</button>

Alternatives considered

No response

Importance

nice to have

Additional Information

https://tanstack.com/start/latest/docs/framework/react/server-functions

const yourFn = createServerFn({ method: 'POST' })
  .validator((formData) => {
    if (!(formData instanceof FormData)) {
      throw new Error('Invalid form data')
    }

    const name = formData.get('name')

    if (!name) {
      throw new Error('Name is required')
    }

    return name
  })
  .handler(async ({ data: name }) => {
    console.log(name) // 'John'
  })

console.info(yourFn.url)

Start by tracing the remote function request path that currently handles JSON payloads and identify where FormData would need to be accepted. Add coverage for text fields, file data, and validation, then verify that a remote command can receive FormData without falling back to manual fetch.