Use MimeTypes guessMimeType for better and more extensible MimeType detection

[Orrison]

Rather than calling for and using Finfo directly, this modifies the Support/File@getMimeType method to instead call Symfony\Component\Mime\MimeTypes@guessMimeType. Since by default the vendor/symfony/mime/FileinfoMimeTypeGuesser is registered last, it will be used. Resulting in the same functionality as before by default with a few added benefits.

First, the default and first Guesser vendor/symfony/mime/FileinfoMimeTypeGuesser, while virtually the same as the original implementation as it calls finfo in almost the same way, should be slightly more robust.

Second, for any reason if the vendor/symfony/mime/FileinfoMimeTypeGuesser is unable to be used or fails to recognize the mime_type the vendor/symfony/mime/FileBinaryMimeTypeGuesser is called.

Finally, the most important benefit, is this allows for extensibility in mime_type detection. A user can register a new Guesser which will be used first.
There are many reasons a user would want to do this, one of the most prominent and my personal use case is making and using a custom Guesser (using getID3) that is better at recognizing mp3s. The default finfo and binary scan, while good for most use-cases, is not great at detecting all mp3s. With this change a User can add a custom Guesser to work how ever they need.

No additional tests were added as tests/Support/FileTest.php should be sufficient as it is still passing after this change. Though I can also add a test for registering a new Guesser if that is wanted.

[freekmurze]

Thank you!