Do not handle a file as application/zip when the extension is not .zip.

chrome/browser/extensions/api/file_handlers/mime_util.cc

Index: chrome/browser/extensions/api/file_handlers/mime_util.cc
diff --git a/chrome/browser/extensions/api/file_handlers/mime_util.cc b/chrome/browser/extensions/api/file_handlers/mime_util.cc
index 74b622c26bdf7ec03b792d0d409553995be20d29..e19f74749c8f588ce9438c568e173c6cd1532aa3 100644
--- a/chrome/browser/extensions/api/file_handlers/mime_util.cc
+++ b/chrome/browser/extensions/api/file_handlers/mime_util.cc
@@ -19,6 +19,12 @@
 
 using content::BrowserThread;
 
+namespace {
+
+const char kMimeTypeApplicationOctetStream[] = "application/octet-stream";
+
+}  // namespace
+
 namespace extensions {
 namespace app_file_handler_util {
 namespace {
@@ -82,6 +88,15 @@ void OnGetMimeTypeFromMetadataForNonNativeLocalPathCompleted(
 void OnSniffMimeTypeForNativeLocalPathCompleted(
     scoped_ptr<std::string> mime_type,
     const base::Callback<void(const std::string&)>& callback) {
+  // Do not return application/zip as sniffed result. If the file has .zip
+  // extension, it should be already returned as application/zip. If the file
+  // does not have .zip extension and couldn't find mime type from the
+  // extension, it might be unknown internally zipped file.
+  if (*mime_type == "application/zip") {
+    callback.Run(kMimeTypeApplicationOctetStream);
+    return;
+  }
+
   callback.Run(*mime_type);
 }
 
@@ -99,7 +114,8 @@ void OnGetMimeTypeFromFileForNativeLocalPathCompleted(
     return;
   }
 
-  scoped_ptr<std::string> sniffed_mime_type(new std::string);
+  scoped_ptr<std::string> sniffed_mime_type(
+      new std::string(kMimeTypeApplicationOctetStream));
   std::string* const sniffed_mime_type_ptr = sniffed_mime_type.get();
   BrowserThread::PostBlockingPoolTaskAndReply(
       FROM_HERE,