Function build_attachment_init 

pub fn build_attachment_init(
    sender: &str,
    receiver: &str,
    attachment_id: Uuid,
    kind: AttachmentKind,
    filename: Option<&str>,
    content_type: Option<&str>,
    total_size: u64,
    chunk_size: u32,
    chunk_count: u32,
    timestamp_ms: u64,
) -> Message