Maybe following function is what you are looking for.
The important part you are looking for in the code is the line which holds the pathinfo,
which is php and not WordPress specific.
There are probably several other options but as nobody responded till now I think that this function will help you out till another (maybe better) answer is added by someone else.
You could make a backup of the functions.php (found in the theme folder) before you add this function to it.
I have tested it in a sandbox with the WP version 4.9.6 and it should work flawless.
* Add content on Alt/Title tags
* @link https://wordpress.stackexchange.com/q/306250
* @version Wordpress V4.9.6
* @see https://codex.wordpress.org/Function_Reference/wp_get_attachment_url
* @see https://secure.php.net/manual/en/function.pathinfo.php
* @see https://secure.php.net/manual/en/function.preg-replace.php
* @param [type] $content [description]
* @return [type] [description]
add_filter( 'the_content', 'add_filename_on_img_tags' );
function add_filename_on_img_tags( $content )
// get featured image
$url = wp_get_attachment_url( get_post_thumbnail_id( $post->ID ) );
// get filename
$filename = pathinfo( $arr['name'], PATHINFO_FILENAME );
// add content on ALT/TITLE tags
$img = '<img src="' . $url . '" alt="' . $filename . '" title="' . $filename . '"/>';
// add image after first paragraph
$content = preg_replace( '#(<p>.*?</p>)#','$1' . $img, $content, 1 );
} // end function
In the docblock you find links with information for code as used in the function.