Allow content-negotiation for POST requests too

[coyotte508]

Describe the problem

I want to do a +server.ts with a POST handler that downloads a file (a JSON backup).

And I want it to be accessible without client-side javascript:

<!-- backup/+page.svelte -->
<form method="POST" action="/backup">
  <input type="submit">
</form>

// backup/+server.ts

export const POST = () => {
  return new Response(JSON.stringify('{}'), {
    headers: {
      'Content-Type': 'application/json',
      'Content-Disposition': 'attachment; filename="backup.json"'
    }
  })
}

Unfortunately, there is a 405 error message with the message:

POST method not allowed. No actions exist for this page

This, despite the lack of +page.server.ts / actions.

Describe the proposed solution

When there is no actions for a page, allow the POST from the native browser form to hit +server.ts's POST handler.

This would be consistent with the GET behavior.

Alternatives considered

Currently, workarounds are using client-side javascript, or redirecting to a GET page in the action's handler.

A possible alternative feature is allow downloading files in an action handler.

Importance

nice to have

Additional Information

No response

[eltigerchino]

The content negotiation seems to be working as intended, but our route handling logic doesn't check if any actions exist first before skipping past our endpoints.

On a side note, this is what the accept header look like for a native form submission request:

text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.7