How to upload file in Symfony LiveComponent with other form data?

It's hard to answer your question without seeing the full code of the UX Live Component, just to illustrate how I use it (regular form submission + file):

<?php

final class UXLiveComponentExample extends AbstractController
{
    use DefaultActionTrait;
    use ComponentWithFormTrait;
    use ComponentToolsTrait;

    #[LiveProp]
    public ?OtherData $otherData = null;

    #[LiveProp(writable: true)]
    public ?string $screenshot = null;

    protected function instantiateForm(): FormInterface
    {
        return $this->createForm(TheFormType::class, $this->otherData);
    }

    #[LiveAction]
    public function upload(Request $request): void
    {
        $file = $request->files->get('block_screenshot');
// Do what you want with the file.
    }

    #[LiveAction]
    public function save(
        EntityManagerInterface $entityManager,
    ): void
    {
        $this->submitForm();
        $form = $this->getForm();

        /** @var OtherData $otherData */
        $otherData = $form->getData();

        // Only update the screenshot if a new one was uploaded
        if ($this->screenshot && $this->screenshot !== $otherData->getScreenshot()) {
            $otherData->setScreenshot($this->screenshot);
        }

        $entityManager->persist($otherData);
        $entityManager->flush();

        // Done.
    }
}

And the Twig side of things:

<div{{ attributes }}>
    {{ form_start(form) }}
        <label for="block_screenshot">Upload Screenshot</label>
        <input name="block_screenshot" id="block_screenshot_id" type="file">
        <button type="button" data-action="live#action" data-live-action-param="files|upload">Upload screenshot</button>

        {{ form_rest(form) }} <!-- This is where all other form fields are rendered. -->
    {{ form_end(form) }}
</div>