Export: an unknown format is an error instead of a deckcraft file

file.export with an explicit format outside png, jpeg, jpg, pdf, pptx, potx,
ppsx, deckcraft, outline and txt fell through to the native deckcraft zip, so a
request for "docx" reported success and wrote a deckcraft archive under that
name. It now returns an error and writes nothing. A format inferred from the
file extension behaves as before.

crates/engine/src/cmd/file.rs

@@ -264,11 +264,12 @@ fn export(s: &mut Session, p: &Value) -> Result<Value> {
             write_file(&path, &bytes)?;
             Ok(json!({"path": path, "bytes": bytes.len()}))
         }
-        other => {
-            let bytes = save_bytes(&st.doc, other)?;
+        "pptx" | "potx" | "ppsx" | "deckcraft" | "outline" | "txt" => {
+            let bytes = save_bytes(&st.doc, &format)?;
             write_file(&path, &bytes)?;
             Ok(json!({"path": path, "bytes": bytes.len()}))
         }
+        other => Err(bad("file.export", format!("unknown format `{other}`"))),
     }
 }