Don't assume that file binary exists on *nix OS

Certain lightweight distributions such as Alpine Linux (popular for smaller Docker images) do not include it by default.

src/Symfony/Component/HttpFoundation/File/MimeType/FileBinaryMimeTypeGuesser.php

@@ -43,7 +43,33 @@ public function __construct($cmd = 'file -b --mime %s 2>/dev/null')
      */
     public static function isSupported()
     {
-        return '\\' !== DIRECTORY_SEPARATOR && function_exists('passthru') && function_exists('escapeshellarg');
+        static $supported;
+
+        if (isset($supported)) {
+            return $supported;
+        }
+
+        if ('\\' === DIRECTORY_SEPARATOR || !function_exists('passthru') || !function_exists('escapeshellarg')) {
+            return $supported = false;
+        }
+
+        ob_start();
+
+        passthru('command -v file', $return);
+        if ($return > 0) {
+            ob_end_clean();
+
+            return $supported = false;
+        }
+
+        $binPath = trim(ob_get_clean());
+
+        if (!preg_match('#^/[^\0/]#', $binPath)) {
+            // it's not a bin path, but an error message
+            return $supported = false;
+        }
+
+        return $supported = true;
     }
 
     /**