Add Content-ID header for attachments

src/lib.rs

@@ -42,6 +42,9 @@ pub mod error;
 pub mod message;
 pub mod transport;
 
+#[macro_use]
+extern crate hyperx;
+
 use crate::error::Error;
 #[cfg(feature = "builder")]
 pub use crate::message::{

src/message/header/content.rs

@@ -7,6 +7,8 @@ use std::{
     str::{from_utf8, FromStr},
 };
 
+header! { (ContentId, "Content-ID") => [String] }
+
 #[derive(Debug, Clone, Copy, PartialEq)]
 pub enum ContentTransferEncoding {
     SevenBit,