WordPress; File renaming on upload - php

Question

I'm trying to rename files while uploading them to WordPress and I want them to get the name of sanitized post title.
Basically I want to do same thing as here, but unfortunately when I use the code from this answer - I don't get the value of $post variable.
The only thing I get is "empty" name with some numbers at the end and the file extension, e.g. "-5263.png", which increses with every new file.
For some reason I don't get the $post value which would give me the post title and it just changes the file name to... well, nothing and just adding some numbers at the end, so it doesn't overrite any other file.
I really would like to know what's wrong with my code:
function new_filename( $filename, $filename_raw ) {
global $post;
$info = pathinfo( $filename );
$ext = empty( $info['extension'] ) ? '' : '.' . $info['extension'];
$new = $post->post_title;
if ( $new != $filename_raw ) {
$new = sanitize_file_name( $new );
}
return $new . $ext;
}
add_filter( 'sanitize_file_name', 'new_filename', 10 );
Thank you in advance for your help.

Answer 1

I've made a plugin a long time ago called File Renaming on Upload that can help you on this, but if you are searching for help with your code i can say that you can try a different approach to get the post variable. Try this instead:
function get_post() {
global $post;
$post_obj = null;
if( !$post ){
$post_id = isset( $_REQUEST['post_id'] ) ? $_REQUEST['post_id'] : false;
if ( $post_id && is_numeric( $post_id ) ) {
$post_obj = get_post( $post_id );
}
}else{
$post_obj = $post;
}
return $post_obj;
}
Once you get your post variable, you don't need to use the post_title like that. You can use
$post->post_name
And then you don't need to use sanitize_file_name() function