diff --git a/src/redux/thunks/viewer.ts b/src/redux/thunks/viewer.ts
index 50de8dc..6cac49c 100644
--- a/src/redux/thunks/viewer.ts
+++ b/src/redux/thunks/viewer.ts
@@ -34,6 +34,7 @@ import { Viewers, ViewersByID } from "../siteConfigSlice.ts";
 import { AppThunk } from "../store.ts";
 import { askSaveAs, askStaleVersionAction } from "./dialog.ts";
 import { longRunningTaskWithSnackbar, refreshSingleFileSymbolicLinks } from "./file.ts";
+import { base64Encode } from "../../util/base64.ts";
 
 export interface ExpandedViewerSetting {
   [key: string]: Viewer[];
@@ -273,7 +274,7 @@ export function openCustomViewer(file: FileResponse, viewer: Viewer, preferredVe
     const vars: { [key: string]: string } = {
       src: encodeURIComponent(entityUrl.urls[0].url),
       src_raw: entityUrl.urls[0].url,
-      src_raw_base64: btoa(entityUrl.urls[0].url),
+      src_raw_base64: base64Encode(entityUrl.urls[0].url),
       name: encodeURIComponent(file.name),
       version: preferredVersion ? preferredVersion : "",
       id: file.id,
diff --git a/src/util/base64.ts b/src/util/base64.ts
new file mode 100644
index 0000000..d833870
--- /dev/null
+++ b/src/util/base64.ts
@@ -0,0 +1,13 @@
+export function base64Encode(data: string | Uint8Array): string {
+  const bytes = typeof data === "string" ? new TextEncoder().encode(data) : data;
+  const binary = Array.from(bytes, (b) => String.fromCharCode(b)).join("");
+  return btoa(binary);
+}
+
+export function base64Decode(data: string): Uint8Array {
+  return Uint8Array.from(atob(data), (c) => c.charCodeAt(0));
+}
+
+export function base64DecodeToString(data: string): string {
+  return new TextDecoder().decode(base64Decode(data));
+}