Validation to accept only GIF images

[ashalathapalagiri]

I have configured kcfinder with ckeditor.Need to set an validation to accept only gif images.It should not accept any png,jpeg format images.
configuration of kcfinder in ckeditor (config.js)

config.filebrowserBrowseUrl = '/kcfinder/browse.php?opener=ckeditor&type=files';
config.filebrowserImageBrowseUrl = '/kcfinder/browse.php?opener=ckeditor&type=images';
config.filebrowserFlashBrowseUrl = '/kcfinder/browse.php?opener=ckeditor&type=flash';
config.filebrowserUploadUrl = '/kcfinder/upload.php?opener=ckeditor&type=files';
config.filebrowserImageUploadUrl = '/kcfinder/upload.php?opener=ckeditor&type=images';
config.filebrowserFlashUploadUrl = '/kcfinder/upload.php?opener=ckeditor&type=flash';

As i have tried by adding these lines in config.php but it doesn't work.

'types' => array(
// The folowing directory types are just for an example
'files' => "",
'flash' => "swf",
'media' => "swf flv avi mpg mpeg qt mov wmv asf rm",
'misc' => "! pdf doc docx xls xlsx",
'images' => "*img",
'mimages' => "*mime image/gif image/png image/jpeg",
'notimages' => "*mime ! image/gif image/png image/jpeg"
),

Can anyone tell me how to validate it to accept only gif images.