File uploads

When the form data contains a File, Inertia sends the request as multipart/form-data. Yii handles it natively:

const { data, setData, post, progress } = useForm({ name: '', avatar: null })

<input type="file" onChange={(e) => setData('avatar', e.target.files[0])} />
{progress && <progress value={progress.percentage} max="100" />}

post('/profile/avatar')

use yii\web\UploadedFile;

public function actionAvatar(): Response
{
    $model = new AvatarForm();
    $model->avatar = UploadedFile::getInstanceByName('avatar');

    if (!$model->upload()) {
        Inertia::withErrors($model);

        return Inertia::back();
    }

    return $this->redirect(['profile/index']);
}

Uploads with PUT or PATCH

PHP only parses multipart/form-data bodies for POST requests. Send a POST and spoof the method with the _method field, which Yii reads through Request::$methodParam:

router.post(`/users/${user.id}`, {
  _method: 'put',
  avatar: file,
})

The action then sees a PUT request and UploadedFile::getInstanceByName('avatar') works as usual.