Rename attachments during upload

Here’s the function I use for WP to rename images during upload on the fly and set the image’s filename to match the post slug.

function wpsx_5505_modify_uploaded_file_names($arr) {

// Get the parent post ID, if there is one
if( isset($_REQUEST['post_id']) ) {
    $post_id = $_REQUEST['post_id'];
} else {
    $post_id = false;
}

// Only do this if we got the post ID--otherwise they're probably in
//  the media section rather than uploading an image from a post.
if($post_id && is_numeric($post_id)) {

    // Get the post slug
    $post_obj = get_post($post_id); 
    $post_slug = $post_obj->post_name;

    // If we found a slug
    if($post_slug) {

        $random_number = rand(10000,99999);
        $arr['name'] = $post_slug . '-' . $random_number . '.jpg';

    }

}

return $arr;

}
add_filter('wp_handle_upload_prefilter', 'wpsx_5505_modify_uploaded_file_names', 1, 1);

I want to keep the original filename adding $ post_slug

[thread_title]-[original_filename].ext

You can try to replace

$arr['name'] = $post_slug . '-' . $random_number . '.jpg';

with

$arr['name'] = $post_slug . '-' . $arr['name'];

to get the file format [post_slug]-[original_filename].ext.

Update:

Here is an example of the $arr structure for an image with the filename car.png :

 Array
(
    [name] => car.png
    [type] => image/png
    [tmp_name] => /tmp/phpJKhCwI
    [error] => 0
    [size] => 5868
)

To get the [post_slug].ext format, one could use this:

$arr['name'] = $post_slug . '.' . pathinfo( $arr['name'], PATHINFO_EXTENSION );

When the post title is My favorite car it becomes:

 Array
(
    [name] => my-favorite-car.png
    [type] => image/png
    [tmp_name] => /tmp/phpJKhCwI
    [error] => 0
    [size] => 5868
)

When more than one image is uploaded with the same filename, the uploaded image filename will become:

my-favorite-car.png  (1. upload)
my-favorite-car1.png (2. upload)
my-favorite-car2.png (3. upload)
...