Implement Blob.close().

LayoutTests/http/tests/xmlhttprequest/resources/multipart-post-echo.php

 <?php
 if (strpos($_SERVER['CONTENT_TYPE'], 'multipart/form-data; boundary=') != 0) {
     echo 'Invalid Content-Types.';
     return;
 }
 
 $values = array();
 
 foreach ($_POST as $key => $value) {
     $values[] = "$key=$value";
 }
 
 foreach ($_FILES as $key => $value) {
     $file = $_FILES[$key];
     if ($file['error']) {
         echo 'Upload file error: ' . $file['error'];
         return;
     } else {
         $fp = fopen($file['tmp_name'], 'r');
         if ($fp) {
-            $content = fread($fp, $file['size']);
+            $content = $file['size'] > 0 ? fread($fp, $file['size']) : "";
             fclose($fp);
         }
         $values[] = $key . '=' . $file['name'] . ':' . $file['type'] . ':' . $content;
     }
 }
 
 echo join('&', $values);
 ?>