wp_generate_attachment_metadata returning an empty array

[admin]

I'm writing a Wordpress plugin to download remote images on my blog.

I made a function to upload a remote image locally, then returning its ID.
All seems OK except that

    $attach_data = wp_generate_attachment_metadata( $attach_id, $local_file );

returns me an empty array - and it should not.

<a href="http://codex.wordpress.org/Function_Reference/wp_generate_attachment_metadata" rel="nofollow">wp_generate_attachment_metadata</a> is, among others, responsible of generating the thumbnails of the uploaded image. But I have no thumbnails created when I run my code.

I checked the values I send to the function and they seems correct : I have an ID and an absolute path to the uploaded file, as documented in the codex.
Still, I can't manage to have my code working :

<strong>$attach_data</strong> should not be empty...

Can anyone help ?

function upload_from_remote_url($url,$post_id){

    $url = $this->validate_remote_media_url($url); //check file is not on local server
    if (!$url) return false;

    if ($existing_id = $this->media_already_exists($url)) return $existing_id; //url already has been downloaded

    $upload_dir = wp_upload_dir();
    $wp_mime_types = wp_get_mime_types();

    //fetch image
    $response = wp_remote_get( $url );

    //get filename without extension
    $filename = basename( $url ); //get filename & extension
    $filename_strip = preg_replace('/\.[^.]*$/', '', $filename); //strip extension

    //get extension from content type,
    //because wp_upload_bits needs an extension and certain url don't have one.

    $file_type = wp_remote_retrieve_header( $response, 'content-type' );
    $extensions = array_search($file_type,$wp_mime_types);
    $extensions_arr = explode('|',$extensions);
    $extension = $extensions_arr[0];

    $new_filename = $filename_strip.'.'.$extension; //full name
    $new_filename = wp_unique_filename($upload_dir['path'], $new_filename); // be sure this name do not exist already

    $uploaded = wp_upload_bits($new_filename, '', wp_remote_retrieve_body( $response ) );
    if ($uploaded['error']) return false;

    $local_file = $uploaded['file'];
    $local_filename = basename($local_file);
    $local_filetype = wp_check_filetype( $local_filename, null );

    //Attachment options
    $attachment = array(
        'post_title'=> $local_filename,
        'post_mime_type' => $local_filetype,
        'post_status' => 'inherit'
    );

    // Add the image to your media library
    $attach_id = wp_insert_attachment( $attachment, $local_file, $post_id );

    if (!$attach_id) return false;

    $attach_data = wp_generate_attachment_metadata( $attach_id, $local_file );

    wp_update_attachment_metadata( $attach_id, $attach_data );

    //save source link so we do not import several times the same media
    update_post_meta($attach_id, 'grm_source', $url);

    return $attach_id;

}

BTW, if any WP gourou had anything to say about this code... I'll be happy to read it, as the WP documentation about uploading files is a bit messy. I needed some specific stuff here, as being able to retrieve the file extension. I ended up to this but maybe you have some better ideas !