[form-data-parser] Accept FormData compatible data types from the upload handler

[lennerd]

There is a minor roadblock in using the multipart-parser library, which seems solvable with a simple fix.

Our Setup

• A user profile can be updated via a form in Remix, including a profile picture.
• We use two backends: one within Remix and another as a Spring application.
• When a file is uploaded from Remix, it’s sent to an S3 bucket via the Spring application.
• The Spring application generates a unique ID for the uploaded file and returns it once the upload is successful. (This ID is stored in the database to reference the file for future access.)
• After receiving the file ID, we replace the file in the form data with its corresponding ID.
• This approach allows us to seamlessly pass the form data along to the Spring application

The Issue

The requestHandler function's upload handler currently only accepts File, null or void as a return type.

Proposed Fix

export interface FileUploadHandler {
  (file: FileUpload): void | null | File | string | Promise<void | null | File | string>;
}

This update allows the handler to return a File, string, null, void, or a Promise resolving to any of these types FormData accepts.

I’m happy to create a PR for this if that would be helpful.

[mjackson]

This makes a ton of sense to me. I'd be happy to merge a PR if you'd like to prepare one. Thank you! 🙏

[lennerd]

Perfect. I’m a bit busy with a deadline right now. I’ll try to get to it over the weekend.