diff --git a/src/common/file/browser.ts b/src/common/file/browser.ts
index 529f7a3..852a88a 100644
--- a/src/common/file/browser.ts
+++ b/src/common/file/browser.ts
@@ -55,6 +55,10 @@ async function fetch_resource_inner(options: DownloadOptions, path: string, prog
     let url: URL | string
     try {
         url = new URL(path, lib_path)
+        const lastmod = options.engine_files?.[path]
+        if (lastmod) {
+            url.searchParams.set("lastmod", lastmod)
+        }
     }
     catch {
         url = lib_path + path
diff --git a/src/common/file/interface.ts b/src/common/file/interface.ts
index ea49e68..464e344 100644
--- a/src/common/file/interface.ts
+++ b/src/common/file/interface.ts
@@ -25,4 +25,5 @@ export interface DownloadOptions {
     /** Use the file proxy; if disabled may mean that some files can't be loaded */
     // We could just say to exclude proxy_url instead?
     use_proxy?: boolean | number,
-}
\ No newline at end of file
+    engine_files?: {[key: string]: string},
+}