set group read permissions by default

src/nex/attachment.rs

@@ -59,6 +59,11 @@ impl Attachment {
             }
             _ => panic!(), // warn as unexpected
         }
+        #[cfg(any(target_os = "linux", target_os = "macos"))]
+        {
+            use std::{fs::Permissions, os::unix::fs::PermissionsExt};
+            fs::set_permissions(target, Permissions::from_mode(0o640))?; // @TODO optional
+        }
         Ok(())
     }
 }