Fix mime-type guesser for Drupal >= 10.3.2

Closes #3486396

src/File/MimeType/HttpMimeTypeGuesser.php

@@ -55,9 +55,8 @@ class HttpMimeTypeGuesser implements MimeTypeGuesserInterface {
     if ($filename = $this->parseFileNameFromUrl($path)) {
       $mimetype = $this->extensionGuesser->guessMimeType($filename);
 
-      if ($mimetype !== 'application/octet-stream') {
-        // Only return the guessed mime type if it found a valid match
-        // instead of returning the default mime type.
+      if ($mimetype !== NULL) {
+        // Only return the guessed mime type if it found a valid match.
         return $mimetype;
       }
     }