How to show file type of featured image?

If you want to show the File type of featured image. then try the below code.

<?php
    $id1 = get_post_thumbnail_id($post->ID);
    $type =  get_post_mime_type( $id1 );
    $mime_type = explode("https://wordpress.stackexchange.com/", $type);
    $type=".".$mime_type['1'];
    echo $type;
?>

It will display the exact extension that you want “.jpeg” from “image/jpeg”
Here “$post->ID” is current post id (or the id of the post whose featured image you want ).
Check if it works for you.