Rename attachments during upload

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)
...