on windows open resources will be locked while the stream is open.
closing the resource allows deletion below
$source = $this->fopen($path1 . $postFix1, 'r');
$target = $this->fopen($path2 . $postFix2, 'w');
list($count, $result) = \OC_Helper::streamCopy($source, $target);
+
+ // close open handle - especially $source is necessary because unlink below will
+ // throw an exception on windows because the file is locked
+ fclose($source);
+ fclose($target);
+
if ($result !== false) {
list($storage1, $internalPath1) = Filesystem::resolvePath($absolutePath1 . $postFix1);
$storage1->unlink($internalPath1);