Empty `file` inputs get submitted as `undefined`

[abdelfattahradwan]

Describe the bug

If you have a form (enhanced or otherwise) that includes a File field (i.e. input[type=file]), if you do not select the file, instead of omitting the field on submission, the field is still included in the submission as undefined. That causes issues with validation libraries that treat optional and undefinable keys differently, for example, ArkType.

Reproduction

https://stackblitz.com/edit/arktype-form-validation-bug?file=src%2Froutes%2F%2Bpage.svelte

Simply, go to the link above and press both "Submit" buttons without selecting files.

Expected:

• Both form submissions should succeed and log {} on the server.

Actual:

• The ArkType form submission fails as file is present but given undefined as a value.
• The Valibot form submission succeeds, but it logs { file: undefined } when it should be an empty object.

Logs

System Info

System:
    OS: Windows 11 10.0.26200
    CPU: (28) x64 Intel(R) Core(TM) i7-14700K
    Memory: 40.85 GB / 63.70 GB
  Binaries:
    Node: 24.16.0 - C:\Program Files\nodejs\node.EXE
    npm: 11.7.0 - ~\AppData\Roaming\npm\npm.CMD
    bun: 1.3.14 - ~\.bun\bin\bun.EXE
  Browsers:
    Edge: Chromium (140.0.3485.81)
    Internet Explorer: 11.0.26100.8115
  npmPackages:
    @sveltejs/adapter-node: 5.5.4 => 5.5.4
    @sveltejs/kit: 2.60.1 => 2.60.1
    @sveltejs/vite-plugin-svelte: 7.1.2 => 7.1.2
    svelte: 5.55.9 => 5.55.9
    vite: 8.0.14 => 8.0.14

Severity

serious, but I can work around it

Additional Information

I chose "serious, but I can work around it" here because it prevents the usage of ArkType in similar cases and could likely cause other issues due to the unexpected object shape. The workaround is to do validation manually.

Also, I cannot, for example, use this:

const MyArkTypeFormDataSchema = type({
  "file?": "File | undefined"
})

As that causes the form to complain with errors like:

Types of property 'input' are incompatible.
  Type '{ file?: File | undefined; }' is not assignable to type 'RemoteFormInput'.
	Property '"file?"' is incompatible with index signature.
	  Type 'File | undefined' is not assignable to type 'MaybeArray<string | number | boolean | File | RemoteFormInput>'.
		Type 'undefined' is not assignable to type 'MaybeArray<string | number | boolean | File | RemoteFormInput>'.ts(2769)