guessMimeType return application/octet-stream if the temporary file don't have a extension

Problem/Motivation

The problem is that Drupal's ExtensionMimeTypeGuesser method cannot detect the MIME types of temporary files, as they do not have a meaningful extension.
The format of the file path
/tmp/Fhknp
The guessMimeType() ExtensionMimeTypeGuesser method always returns application/octet-stream, as it cannot find the extension.

Steps to reproduce

Create a Form API form that uses the file render element, and run the following code snippet inside the form validator:
Test to upload pdf file.

$validator = Validation::createValidator();
$upload_file = $form_state->getValue('files')[0];
$violations = $validator->validate($upload_file, [
          new File([
            ‘mimeTypes’ => [
              ‘application/pdf’,
            ],
          ]),
        ]);
foreach ($violations as $violation) {
  $violation_file = $violation->getRoot();
  $form_state->setErrorByName(‘files’, $violation->getMessage());
}

Proposed resolution

Introduce a new FileMimeTypeGuesser service that can detect MIME types based on file contents rather than extensions. This resolves the issue for temporary files. A patch has been provided.