bug #26886 Don't assume that file binary exists on *nix OS (teohhanhui)

This PR was merged into the 2.7 branch.

Discussion
----------

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

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | N/A
| License       | MIT
| Doc PR        | N/A

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

Commits
-------

e2c1f24 Don't assume that file binary exists on *nix OS

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

@@ -43,7 +43,21 @@ 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 = null;
+
+        if (null !== $supported) {
+            return $supported;
+        }
+
+        if ('\\' === DIRECTORY_SEPARATOR || !function_exists('passthru') || !function_exists('escapeshellarg')) {
+            return $supported = false;
+        }
+
+        ob_start();
+        passthru('command -v file', $exitStatus);
+        $binPath = trim(ob_get_clean());
+
+        return $supported = 0 === $exitStatus && '' !== $binPath;
     }
 
     /**