Implement missing multipart/form-data handling in auth-next's service-worker-sessions.

[tyshgc]

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What steps will reproduce the problem:

1. Start the service worker session in the auth-next snippet.
2. Attempt to upload a file using a form with enctype="multipart/form-data".
3. Observe that the session does not handle the multipart/form-data POST request correctly.

What is the expected result?

The service worker session should correctly handle multipart/form-data encoded forms, parsing the file input and passing it along to the server without issues.

What happens instead of that?

The service worker cannot process the multipart/form-data content type and returns buffer information in the body as text data.

The file upload succeeds, but the file is corrupted due to this.