fix: force download for untyped (octet-stream) served blobs

The /files handler served the application/octet-stream fallback with no
Content-Disposition, letting a browser render it inline — a stored-XSS vector if
the blob is crafted HTML/JS. Add Content-Disposition: attachment for that type
(belt to the existing nosniff); known image types stay inline.

Test (new tests/api_files.rs): octet-stream → attachment; png → inline.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/tests/api_files.rs

@@ -0,0 +1,57 @@
+mod common;
+
+use axum::http::StatusCode;
+use tower::ServiceExt;
+
+/// Write a blob straight into the harness storage root at `key`, bypassing the
+/// upload handlers — the only way to land a key whose extension resolves to the
+/// `application/octet-stream` fallback (uploads always mint image extensions).
+fn write_blob(h: &common::Harness, key: &str, bytes: &[u8]) {
+    let path = h._storage_dir.path().join(key);
+    std::fs::create_dir_all(path.parent().unwrap()).unwrap();
+    std::fs::write(path, bytes).unwrap();
+}
+
+#[sqlx::test(migrations = "./migrations")]
+async fn octet_stream_blobs_are_served_as_attachment(pool: sqlx::PgPool) {
+    // A blob with an unknown extension serves as application/octet-stream. Such a
+    // body could be crafted HTML/JS, so it must never render inline: force a
+    // download with Content-Disposition: attachment (nosniff is already set).
+    let h = common::harness(pool);
+    write_blob(&h, "misc/blob.bin", b"\x00\x01not-an-image");
+
+    let resp = h
+        .app
+        .oneshot(common::get("/api/v1/files/misc/blob.bin"))
+        .await
+        .unwrap();
+    assert_eq!(resp.status(), StatusCode::OK);
+    assert_eq!(
+        resp.headers().get("content-type").unwrap(),
+        "application/octet-stream"
+    );
+    assert_eq!(
+        resp.headers().get("content-disposition").unwrap(),
+        "attachment"
+    );
+}
+
+#[sqlx::test(migrations = "./migrations")]
+async fn image_blobs_are_served_inline(pool: sqlx::PgPool) {
+    // Regression guard: known image types keep rendering inline (no attachment
+    // disposition), so covers/pages still display in the reader.
+    let h = common::harness(pool);
+    write_blob(&h, "misc/pic.png", &common::fake_png_bytes());
+
+    let resp = h
+        .app
+        .oneshot(common::get("/api/v1/files/misc/pic.png"))
+        .await
+        .unwrap();
+    assert_eq!(resp.status(), StatusCode::OK);
+    assert_eq!(resp.headers().get("content-type").unwrap(), "image/png");
+    assert!(
+        resp.headers().get("content-disposition").is_none(),
+        "images must render inline, not download"
+    );
+}