PNG upload

Hi,

Attempting to upload a PNG picture(in the image link) and it is not allowing it to upload. I am able to upload other PNG images.
I can't upload picture in codeigniter forum attachment .

Image link : https://drive.google.com/open?id=0Bz2laj...FNrN1RzWXc

This is my image upload code.

PHP Code:

$config['upload_path'] = FCPATH.'public/images/pins/';
 $config['allowed_types'] = 'png|gif|jpg|jpeg';

 $this->load->library('upload', $config);
 if (! $this->upload->do_upload("file")) {
 echo "invalid";
 //echo json_encode($FILES['file']['type']);
 }
 else
 {
 $image=$this->upload->data();
 $image_name=$image['file_name'];
 echo json_encode($image_name);
 } 


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