[6.x] validateDimensions() handle image/svg MIME

[0xb4lint]

Added image/svg MIME support to validateDimensions() function.

During the image dimensions validation there is a getMimeType() function called on a Illuminate\Http\UploadedFile class (inherited from Symfony\Component\HttpFoundation\File\UploadedFile <- Symfony\Component\HttpFoundation\File\File) class.

It utilizes the Symfony\Component\Mime\MimeTypes which calls the guessMimeType() on the Symfony\Component\Mime\FileinfoMimeTypeGuesser class.

At this point finfo class is used:

if (!$finfo = new \finfo(FILEINFO_MIME_TYPE, $this->magicFile)) {
	return null;
}

return $finfo->file($path);

I've found it can return two different values for SVG files: image/svg+xml and image/svg.

I didn't find any documentation about this behavior (only a magic test: https://github.com/php/php-src/blob/master/ext/fileinfo/tests/magic), but it can be easily reproduced via this Gist:
https://gist.github.com/0xB4LINT/0d90ddaa02c91147635667b2a8b90f9b

The only difference between the source files is the lack of <?xml ... ?> row.

By the W3's SVG standard document there are more valid document structures defined:
<?xml ... ?><svg ... or <svg xmlns="...

Extends #30009: replaced getClientMimeType() with getMimeType(), but doesn't handle the image/svg MIME.