server: handle application/* as an attachment

server/src/nm.rs

@@ -479,6 +479,12 @@ pub fn attachment_bytes(nm: &Notmuch, id: &str, idx: &[usize]) -> Result<Attachm
     let file = File::open(&path)?;
     let mmap = unsafe { MmapOptions::new().map(&file)? };
     let m = parse_mail(&mmap)?;
+    if idx.is_empty() {
+        let Some(attachment) = extract_attachment(&m, id, &[]) else {
+            return Err(ServerError::PartNotFound);
+        };
+        return Ok(attachment);
+    }
     if let Some(attachment) = walk_attachments(&m, |sp, cur_idx| {
         if cur_idx == idx {
             let attachment = extract_attachment(&sp, id, idx).unwrap_or(Attachment {