PHP文件上传不工作在centos服务器

这些是我用来上传文件的代码。在Xammp窗口中一切工作正常。但是它不能在centos服务器下工作。它抛出错误"Invalid file"。

upload.php

   <?php
    $allowedExts = array("json");
    $temp = explode(".", $_FILES["file"]["name"]);
    $extension = end($temp);
    if ((($_FILES["file"]["type"] == "application/json"))
          && ($_FILES["file"]["size"] < 20000) && in_array($extension, $allowedExts))
    {
      if ($_FILES["file"]["error"] > 0)
       {
         echo "Return Code: " . $_FILES["file"]["error"] . "<br>";
       }
       else
         {
           echo "Upload: " . $_FILES["file"]["name"] . "<br>";
           echo "Type: " . $_FILES["file"]["type"] . "<br>";
           echo "Size: " . ($_FILES["file"]["size"] / 1024) . " kB<br>";
           echo "Temp file: " . $_FILES["file"]["tmp_name"] . "<br>";
          if (file_exists("uploads/" . $_FILES["file"]["name"]))
            {
             echo $_FILES["file"]["name"] . " already exists. ";
            }
            else
            {
           $dir="uploads";
               if ($handle = opendir($dir)) {
               while (false !== ($entry = readdir($handle))) {
               if ($entry != "." && $entry != "..") {
           unlink("uploads/$entry");
            }
          }
          closedir($handle);
        }             
         move_uploaded_file($_FILES["file"]["tmp_name"],
         $f="uploads/" . $_FILES["file"]["name"]);
     chmod($f, 0777);
         echo "Stored in: " . "uploads/" . $_FILES["file"]["name"];
     header("Location: user.php");
        }
      }
   }
  else
   {
     echo "Invalid file";
   }

  ?>

     <form action="upload.php" method="POST" enctype="multipart/form-data">
     <label for="file"><span style="color:#ffffff;">Upload File:</span></label>
     <input type="file" name="file" id="file">
     <input class="btn btn-success btn-block" type="submit" name="submit" value="Submit">

上传的文件为Json文件，文件大小为1.02kb。

谁来帮我解决这个问题