The opaque content type is one core constant: local::OPAQUE_MIME

"application/octet-stream" was written out in the blob server, its test, the
store's normalize_mime and the wire default. All four now read OPAQUE_MIME; the
schema's SQL column default names the same string and says so.

DRY pass #2, batch 2, F7 (#5372).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

core/src/local/store.rs

@@ -15,7 +15,7 @@ use uuid::Uuid;
 use crate::local::derive;
 use crate::local::models::*;
 use crate::local::recur;
-use crate::local::{iso, now};
+use crate::local::{iso, now, OPAQUE_MIME};
 
 fn new_id() -> String {
     Uuid::new_v4().to_string()
@@ -792,7 +792,7 @@ fn normalize_mime(raw: &str) -> String {
         .trim()
         .to_ascii_lowercase();
     if bare.is_empty() {
-        "application/octet-stream".to_string()
+        OPAQUE_MIME.to_string()
     } else {
         bare
     }