bug symfony#18271 [FileSystem] Google app engine filesystem (swordbeta)

This PR was submitted for the master branch but it was merged into the 2.8 branch instead (closes symfony#18271).

Discussion
----------

[FileSystem] Google app engine filesystem

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | symfony#17922
| License       | MIT
| Doc PR        | -

Commits
-------

06d8697 [FileSystem] Add support for Google App Engine.

Author: fabpot

src/Symfony/Component/Filesystem/Filesystem.php

@@ -486,13 +486,13 @@ public function tempnam($dir, $prefix)
     {
         list($scheme, $hierarchy) = $this->getSchemeAndHierarchy($dir);
 
-        // If no scheme or scheme is "file" create temp file in local filesystem
-        if (null === $scheme || 'file' === $scheme) {
+        // If no scheme or scheme is "file" or "gs" create temp file in local filesystem
+        if (null === $scheme || 'file' === $scheme || 'gs' === $scheme) {
             $tmpFile = tempnam($hierarchy, $prefix);
 
             // If tempnam failed or no scheme return the filename otherwise prepend the scheme
             if (false !== $tmpFile) {
-                if (null !== $scheme) {
+                if (null !== $scheme && 'gs' !== $scheme) {
                     return $scheme.'://'.$tmpFile;
                 }