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..525df80241a24fd197895045f9db7671dbee44e8 100644
--- a/chrome/browser/extensions/api/file_handlers/mime_util.cc
+++ b/chrome/browser/extensions/api/file_handlers/mime_util.cc
@@ -82,6 +82,15 @@ 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("application/octet-stream");
+    return;
+  }
+
   callback.Run(*mime_type);
 }