diff --git a/src/client/stdlib/duckdb.js b/src/client/stdlib/duckdb.js
index 18ce36b36..c4d6093ac 100644
--- a/src/client/stdlib/duckdb.js
+++ b/src/client/stdlib/duckdb.js
@@ -255,8 +255,10 @@ async function insertFile(database, name, file, options) {
         if (/\.parquet$/i.test(file.name)) {
           return await connection.query(`CREATE VIEW '${name}' AS SELECT * FROM parquet_scan('${file.name}')`);
         }
-        // Lastly, let DuckDB guess: it may be a .duckdb (or .db or .ddb) database file.
-        return await connection.query(`ATTACH '${file.name}' AS ${name} (READ_ONLY)`);
+        if (/\.(db|ddb|duckdb)$/i.test(file.name)) {
+          return await connection.query(`ATTACH '${file.name}' AS ${name} (READ_ONLY)`);
+        }
+        throw new Error(`unknown file type: ${file.mimeType}`);
     }
   } finally {
     await connection.close();