* Electron Version: Electron 2.0.0
* Operating System: Windows 10

**Expected Behavior**
When a single file extension is given (`<input type="file" accept=".txt" />`), the format description in the open file dialog should not be empty.

**Actual behavior**
When a single file extension is given (`<input type="file" accept=".txt" />`), the format description in the open file dialog is empty.

**To Reproduce**
- open the devtools console in the Electron default app
- run `document.body.innerHTML = '<input type="file" accept=".txt" />';`
- click the "Choose File" button
- observe the file format selection combo box in the bottom-right corner

**Screenshots**
```
<input type="file" accept=".txt" />
```
![one-file](https://user-images.githubusercontent.com/1281234/40083145-8b2c0d92-5893-11e8-9203-f9631dd440ac.PNG)

```
<input type="file" accept=".txt,.zip" />
```
![two-files](https://user-images.githubusercontent.com/1281234/40083146-8b48bece-5893-11e8-9213-980fceee41f1.PNG)

atom/browser/web_dialog_helper.cc
#include "atom/browser/web_dialog_helper.h"
#include <string>
#include <vector>
#include "atom/browser/atom_browser_context.h"
#include "atom/browser/native_window.h"
#include "atom/browser/ui/file_dialog.h"
#include "base/bind.h"
#include "base/files/file_enumerator.h"
#include "base/files/file_path.h"
#include "base/strings/utf_string_conversions.h"
#include "chrome/common/pref_names.h"
#include "components/prefs/pref_service.h"
#include "content/public/browser/render_frame_host.h"
#include "content/public/browser/render_process_host.h"
#include "content/public/browser/render_view_host.h"
#include "content/public/browser/web_contents.h"
#include "content/public/common/file_chooser_file_info.h"
#include "content/public/common/file_chooser_params.h"
#include "net/base/mime_util.h"
#include "ui/shell_dialogs/selected_file_info.h"
namespace {
class FileSelectHelper : public base::RefCounted<FileSelectHelper>,
                         public content::WebContentsObserver {
 public:
  FileSelectHelper(content::RenderFrameHost* render_frame_host,