Function send_file_dm 

pub async fn send_file_dm(
    receiver_npub: &str,
    file_bytes: Arc<Vec<u8>>,
    filename: &str,
    extension: &str,
    content: Option<&str>,
    config: &SendConfig,
    callback: Arc<dyn SendCallback>,
) -> Result<SendResult, String>

Send a NIP-17 gift-wrapped file attachment DM.

Flow: hash → save locally → encrypt → upload → build Kind 15 rumor → gift-wrap + send.