Media thumbnail for custom post inside post content

@Ibraheem, you don’t have to use global $post, since it is in global. To check post type you can use get_post_type().
Using the_post_thumbnail is not properly implemented in this case, instead use get_the_post_thumbnail. Note: you can’t use has_post_thumbnail as tag condition if you are not set featured image.

add_filter( 'the_content', 'put_thumbnail_in_posting' );
function put_thumbnail_in_posting( $content )
{
    if ( 'advert' == get_post_type() && has_post_thumbnail() )
    {
        $thumbnail = get_the_post_thumbnail( null, $size="", array(
                'style' => 'float:left;margin:15px;'
        ) );

        $content = $thumbnail . $content; //thumbnail in top text
        /* $content = $content . $thumbnail; //thumbnail in bottom text  */
    }

    return $content;
}

To get all attachment in post, you can implement with the following way:

add_filter( 'the_content', 'put_thumbnail_in_posting' );
function put_thumbnail_in_posting( $content )
{
    if ( 'advert' == get_post_type() )
    {
        $args = array(
            'order'          => 'ASC',
            'post_mime_type' => 'image',
            'post_parent'    => get_the_ID(),
            'post_status'    => null,
            'post_type'      => 'attachment',
        );

        $attachments = get_children( $args );

        if ( $attachments ) {
            $thumbnails="";
            foreach( $attachments as $attachment )
            {
                $thumbnails .= wp_get_attachment_image( $attachment->ID, $size = null, $icon = true, array(
                    'style' => 'float:left;margin:15px;'
                ) );
            }
            $content = $thumbnails . $content;
        }
    }
    return $content;
}

You can add another tag condition such as is_home, is_single, etc. and tweak thumb arguments in code that suit with your need.