Minor cleanup of Viewtype

Provide checking for attachment as a method and move it to the message
module.  The method is a lot easier to read and have correct
expectations about.

src/e2ee.rs

@@ -400,15 +400,14 @@ pub async fn ensure_secret_key_exists(context: &Context) -> Result<String> {
 
 #[cfg(test)]
 mod tests {
-    use super::*;
-
     use crate::chat;
-    use crate::constants::Viewtype;
-    use crate::message::Message;
+    use crate::message::{Message, Viewtype};
     use crate::param::Param;
     use crate::peerstate::ToSave;
     use crate::test_utils::{bob_keypair, TestContext};
 
+    use super::*;
+
     mod ensure_secret_key_exists {
         use super::*;