--- a/mobile/components/AutoCompleteCache.js
+++ b/mobile/components/AutoCompleteCache.js
@@ -196,17 +196,19 @@ var AutoCompleteUtils = {
   },
 
   loadCache: function loadCache() {
     if (!this.cacheFile.exists())
       return;
 
     try {
       let self = this;
-      NetUtil.asyncFetch(this.cacheFile, function(aInputStream, aResultCode) {
+      let channel = NetUtil.newChannel(this.cacheFile);
+      channel.setContentType("application/json");
+      NetUtil.asyncFetch(channel, function(aInputStream, aResultCode) {
         if (Components.isSuccessCode(aResultCode)) {
           let cache = Cc["@mozilla.org/dom/json;1"].createInstance(Ci.nsIJSON).
                       decodeFromStream(aInputStream, aInputStream.available());
 
           if (cache.version != CACHE_VERSION) {
             self.fetch(self.query);
             return;
           }