Image upload API works with postman but not working with Unity.

I'm using Unity 2018.3.14f1.

I have created API for uploading image in php. Which is working well in the postman and not working in unity editor, All the time it gives me following log "HTTP/1.1 403 Forbidden".

Here is my code:

 IEnumerator UploadImage()
 {
         yield return new WaitForEndOfFrame();   
         Texture2D screenImage = new Texture2D(Screen.width, Screen.height);
         screenImage.ReadPixels(new Rect(0, 0, Screen.width, Screen.height), 0, 0);
         screenImage.Apply();
         byte[] imageBytes = screenImage.EncodeToPNG();
 
         WWWForm form = new WWWForm();       
         form.AddBinaryData("file", imageBytes, "screenShot.png", "image/png");
 
         using (var w = UnityWebRequest.Post(screenShotURL, form))
         {
             yield return w.SendWebRequest();
             if (w.isNetworkError || w.isHttpError)
             {
                 print(w.error);
             }
             else
             {
                 print("Finished Uploading Screenshot");
             }
         }
 }

Here is php script :

 if ((($_FILES["file"]["type"] == "image/gif") || ($_FILES["file"]["type"] == "image/jpeg") || ($_FILES["file"]["type"] == "image/png")) && ($_FILES["file"]["size"] < 20000000000))
 {
     if ($_FILES["file"]["error"] > 0) {
         //echo "Return Code: " . $_FILES["file"]["error"] . "";
     } else {
         echo "Upload: " . $_FILES["file"]["name"] . "";
         echo "Type: " . $_FILES["file"]["type"] . "";
         echo "Size: " . ($_FILES["file"]["size"] / 1024) . " Kb";
         echo "Temp file: " . $_FILES["file"]["tmp_name"] . "";
         if (file_exists("Images/" . $_FILES["file"]["name"]))
         {
             echo $_FILES["file"]["name"] . " already exists. ";
         }
         else
         {
             move_uploaded_file($_FILES["file"]["tmp_name"], "Images/" . $_FILES["file"]["name"]);
             echo "Stored in: " . "Images/" . $_FILES["file"]["name"];
         }
     }
 } else {
     echo "Invalid file";
 }