* 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
case content::FileChooserParams::UploadFolder:
        flags |= file_dialog::FILE_DIALOG_OPEN_DIRECTORY;
        break;
      default:
        NOTREACHED();
    }
    auto* browser_context = static_cast<atom::AtomBrowserContext*>(
        render_frame_host->GetProcess()->GetBrowserContext());
    settings.default_path = browser_context->prefs()
                                ->GetFilePath(prefs::kSelectFileLastDirectory)
                                .Append(params.default_file_name);
    settings.properties = flags;
    file_select_helper->ShowOpenDialog(settings);
  }
}
void WebDialogHelper::EnumerateDirectory(content::WebContents* web_contents,
                                         int request_id,
                                         const base::FilePath& dir) {
  int types = base::FileEnumerator::FILES | base::FileEnumerator::DIRECTORIES |
              base::FileEnumerator::INCLUDE_DOT_DOT;
  base::FileEnumerator file_enum(dir, false, types);
  base::FilePath path;
  std::vector<base::FilePath> paths;
  while (!(path = file_enum.Next()).empty())
    paths.push_back(path);
  web_contents->GetRenderViewHost()->DirectoryEnumerationFinished(request_id,
                                                                  paths);
}
}