Inconsistent MIME types chosen for private file system

Problem

The FileDownloadController serves files for Drupal's private file system. It issues a Symfony BinaryFileResponse when a file is allowed to be retrieved. During preparation of this response, it utilizes Symfony's MIME type guesser to set the Contenty-Type header. By default, Symfony adds a couple of guessers, but as part of prehandling the response in DrupalKernel, Drupal registers it's own ExtensionMimeTypeGuesser that, like the name suggests, guesses the MIME type based on the extension.

Prior to Symfony 4.3, the MIME guesser was included as part of the Http Foundation component. As part of Symfony 4.3, a new dedicated MIME component was added and the old pieces in Http Foundation were deprecated. So now the problem: at this point, the BinaryFileResponse was also moved to using the new MIME component for guessing, but Drupal is still registering it's extension-based guesser with the deprecated MIME guesser mechanism. As a result, Drupal's guesser is no longer being invoked when private files are downloaded and you can get some weird results from the default Symfony guessers that cause a CSS file to be classified as an assembly file (for instance).

Steps to reproduce

1. Add a CSS file with a rule like .globl-item { display: none; } to the private file system.
2. Download the file and notice the content type is text/x-asm instead of the expected text/css