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/download.rs

@@ -6,12 +6,11 @@ use serde::{Deserialize, Serialize};
 use std::collections::BTreeMap;
 
 use crate::config::Config;
-use crate::constants::Viewtype;
 use crate::context::Context;
 use crate::dc_tools::time;
 use crate::imap::{Imap, ImapActionResult};
 use crate::job::{self, Action, Job, Status};
-use crate::message::{Message, MsgId};
+use crate::message::{Message, MsgId, Viewtype};
 use crate::mimeparser::{MimeMessage, Part};
 use crate::param::Params;
 use crate::{job_try, stock_str, EventType};
@@ -255,13 +254,15 @@ impl MimeMessage {
 
 #[cfg(test)]
 mod tests {
-    use super::*;
+    use num_traits::FromPrimitive;
+
     use crate::chat::send_msg;
-    use crate::constants::Viewtype;
     use crate::dc_receive_imf::dc_receive_imf_inner;
     use crate::ephemeral::Timer;
+    use crate::message::Viewtype;
     use crate::test_utils::TestContext;
-    use num_traits::FromPrimitive;
+
+    use super::*;
 
     #[test]
     fn test_downloadstate_values() {