* 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
const content::FileChooserParams::Mode& mode)
      : render_frame_host_(render_frame_host), mode_(mode) {
    auto* web_contents =
        content::WebContents::FromRenderFrameHost(render_frame_host);
    content::WebContentsObserver::Observe(web_contents);
  }
  void ShowOpenDialog(const file_dialog::DialogSettings& settings) {
    auto callback = base::Bind(&FileSelectHelper::OnOpenDialogDone, this);
    file_dialog::ShowOpenDialog(settings, callback);
  }
  void ShowSaveDialog(const file_dialog::DialogSettings& settings) {
    auto callback = base::Bind(&FileSelectHelper::OnSaveDialogDone, this);
    file_dialog::ShowSaveDialog(settings, callback);
  }
 private:
  friend class base::RefCounted<FileSelectHelper>;
  ~FileSelectHelper() override {}
#if defined(MAS_BUILD)
  void OnOpenDialogDone(bool result,
                        const std::vector<base::FilePath>& paths,
                        const std::vector<std::string>& bookmarks)
#else
  void OnOpenDialogDone(bool result, const std::vector<base::FilePath>& paths)