diff --git a/zundler/assets/inject_post.js b/zundler/assets/inject_post.js
index 6f07045..1173c06 100644
--- a/zundler/assets/inject_post.js
+++ b/zundler/assets/inject_post.js
@@ -64,7 +64,7 @@ var retrieve_file = function(path) {
         console.warn("File not found: " + path);
         return "";
     } else {
-        return file.data;
+        return file;
     }
 };
 
diff --git a/zundler/assets/inject_pre.js b/zundler/assets/inject_pre.js
index e4d5013..7cfc151 100644
--- a/zundler/assets/inject_pre.js
+++ b/zundler/assets/inject_pre.js
@@ -49,14 +49,55 @@ window.history.replaceState = myReplaceState;
 
 const { fetch: originalFetch } = window;
 
+async function waitFor(predicate, timeout) {
+  return new Promise((resolve, reject) => {
+    const check = () => {
+      console.log('checking', predicate());
+      if (!predicate()) return;
+      clearInterval(interval);
+      resolve();
+    };
+    const interval = setInterval(check, 100);
+    check();
+
+    if (!timeout) return;
+    setTimeout(() => {
+      clearInterval(interval);
+      reject();
+    }, timeout);
+  });
+}
+
+var _base64ToArrayBuffer = function (base64) {
+    if (!base64) { return []}
+    var binary_string = window.atob(base64);
+    var len = binary_string.length;
+    var bytes = new Uint8Array(len);
+    for (var i = 0; i < len; i++) {
+        bytes[i] = binary_string.charCodeAt(i);
+    }
+    return bytes.buffer;
+};
+
 window.fetch = async (...args) => {
+    // wait until global_context is ready
+    try {
+        await waitFor(() => window.hasOwnProperty("global_context"), 10000);
+    } catch (err) {
+        throw err;
+    }
+
     let [resource, config ] = args;
     var path = normalize_path(resource);
     var response;
     if (is_virtual(path)) {
-        var data = retrieve_file(path);
+        var file = retrieve_file(path);
+        var data = file.data;
+        if (file.base64encoded) {
+            data = _base64ToArrayBuffer(data);
+        }
         response = new Response(data);
-
+        response.headers.set("content-type", file.mime_type);
     } else {
         response = await originalFetch(resource, config);
     }
diff --git a/zundler/assets/main.js b/zundler/assets/main.js
index 837e628..de371b3 100644
--- a/zundler/assets/main.js
+++ b/zundler/assets/main.js
@@ -39,7 +39,7 @@ var retrieve_file = function(path) {
         console.warn("File not found: " + path);
         return "";
     } else {
-        return file.data;
+        return file;
     }
 };
 
@@ -91,7 +91,7 @@ var embed_js = function(doc) {
                 let [path, get_parameters, anchor] = split_url(src);
                 path = normalize_path(path);
                 console.debug("Embed script: " + path);
-                var src = retrieve_file(path) + ' \n//# sourceURL=' + path;
+                var src = retrieve_file(path).data + ' \n//# sourceURL=' + path;
                 newScript.appendChild(doc.createTextNode(src));
                 newScript.removeAttribute('src');
                 oldScript.parentNode.replaceChild(newScript, oldScript);
@@ -111,7 +111,7 @@ var embed_css = function(doc) {
             var href = link.getAttribute('href');
             let [path, get_parameters, anchor] = split_url(href);
             path = normalize_path(path);
-            style.textContent = retrieve_file(path);
+            style.textContent = retrieve_file(path).data;
             link.replaceWith(style);
         };
     });
@@ -161,11 +161,11 @@ var embed_img = function(img) {
         if (is_virtual(src)) {
             var path = normalize_path(src);
             const file = retrieve_file(path);
-            const mime_type = window.global_context.file_tree[path].mime_type;
+            const mime_type = file.mime_type;
             if (mime_type == 'image/svg+xml') {
-                img.setAttribute('src', "data:image/svg+xml;charset=utf-8;base64, " + btoa(file));
+                img.setAttribute('src', "data:image/svg+xml;charset=utf-8;base64, " + btoa(file.data));
             } else {
-                img.setAttribute('src', `data:${mime_type};base64, ${file}`);
+                img.setAttribute('src', `data:${mime_type};base64, ${file.data}`);
             }
         };
     };
diff --git a/zundler/embed.py b/zundler/embed.py
index e0a12c4..0f431f8 100644
--- a/zundler/embed.py
+++ b/zundler/embed.py
@@ -145,9 +145,10 @@ def prepare_file(filename, before, after):
         "ttf",
         "gif",
         "ico",
+        "wasm",
     ]:
         # JSON doesn't allow binary data
-        data = base64.b64encode(data)
+        data = base64.b64encode(data).decode()
         base64encoded = True
 
     elif ext in ["html", "htm"]:
@@ -163,6 +164,7 @@ def prepare_file(filename, before, after):
             data = data.decode()
         except UnicodeError:
             data = base64.b64encode(data).decode()
+            base64encoded = True
 
     logger.debug("loaded file: %s [%s, %d bytes]" % (filename, mime_type, len(data)))