Upload PDF file (not required)

I have problem with form validation for upload file, here my validation rule

'file' => [
              // 'uploaded[file]',
                'mime_in[file,application/pdf,image/jpg,image/jpeg,image/gif,image/png]',
                'max_size[file,4096]',
    ],


However, if I don't upload the image, it shows validation error. How to set this validation rule only to apply to an uploaded file?