File format of post image attachment

$post_thumbnail_id = get_post_thumbnail_id();
$thumb = get_post($post_thumbnail_id);
var_dump($thumb);

Look at the post_mime_type in the $thumb object.

Reference

http://codex.wordpress.org/Function_Reference/get_the_post_thumbnail
http://codex.wordpress.org/Function_Reference/get_post

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