fix: allow setting files binding for <input type="file" /> (Svelte 4)

[eltigerchino]

fixes #8316

This PR allows setting the files binding for file inputs to allow for common operations such as setting the files to a drag and drop file, or clearing the file input selection. Currently, this is not possible because bind:files is read-only in Svelte. The redundant alternative is to bind the input element instead and set the files property there.

<script>
  let files;
</script>

<input type="file" bind:files />

<!-- Reset file input -->
<button on:click={() => files = new DataTransfer().files}>Reset</button>

<!-- Set file input as the dragged file -->
<svelte:window
	on:drop|preventDefault={(event) => {
		const { dataTransfer } = event;
		if (!dataTransfer) return;

		files = dataTransfer.files;
	}}
	on:dragover|preventDefault
/>

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• Prefix your PR title with feat:, fix:, chore:, or docs:.
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint

[changeset-bot]

🦋 Changeset detected

Latest commit: 1b2dba6

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
svelte	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[benmccann]

@s3812497 it looks like this PR has a merge conflict

[eltigerchino]

resolved the merge conflict

[dummdidumm]

#10032 revealed that this would actually be a breaking change, therefore closing