login as different user

Capture.jpg is the normal alert list in alert block, can display icon ------A
(sorry only can upload one attachment, but A is the alerts with the icon display)

Capture2.jpg can't display gif icon. -------B

so i copy the image url: http://ilearn-uat.ida.gov.sg/local/icon.php?type=msg&icon=learningplan-request

like the capture images, A can display icon, B can't

so i check the codes..................all the same until the function send_file, in lib/filelib.php, line 800

 else {
            @header('Content-Type: '.$mimetype);
        }

I echo $mimetype;die; before @header. both display image/gif

I echo $mimetype;die; after @header, A can display image/gif, so A can continue to display the image,
but B can't echo $mimetype, because @header return someting error, so the function is end. and it display error image icon.

why some user can display but some not? how can i fix this issue, sorry for poor english