* Electron version: 1.4.15 (probably older versions too).
* Operating system: macOS 10.12.2

When Quick Look is launched by right-clicking a file in a file dialog, clicking its close button doesn't work (and there's an audible beep), and after enough clicking around frequently it crashes. I suspect the same thing would happen using `dialog.showOpenDialog` presented as a sheet, but I haven't tested.

Minimal app to reproduce:
```js
const {app, BrowserWindow} = require('electron')

app.on('ready', () => {
  const win = new BrowserWindow({width: 300, height: 100})
  win.loadURL('data:text/html,<input type="file">')
})
```

Example:
![quicklook](https://cloud.githubusercontent.com/assets/5355/22160125/06120964-df12-11e6-86db-996f2233bf5e.gif)

Crash log: https://gist.github.com/javan/4c19dddd73b0581e60f6baf3b75d4dd8
atom/browser/web_dialog_helper.cc
for (const auto& accept_type : accept_types) {
    std::string ascii_type = base::UTF16ToASCII(accept_type);
    if (ascii_type[0] == '.') {
      
      
      base::FilePath::StringType extension(
          ascii_type.begin(), ascii_type.end());
      
      extensions.push_back(extension.substr(1));
    } else {
      
      net::GetExtensionsForMimeType(ascii_type, &extensions);
    }
  }
  
  if (extensions.empty())
    return filters;
  filters.push_back(file_dialog::Filter());
  for (const auto& extension : extensions) {
#if defined(OS_WIN)
    filters[0].second.push_back(base::UTF16ToASCII(extension));
#else
    filters[0].second.push_back(extension);
#endif
  }
  return filters;
}
}  
namespace atom {
WebDialogHelper::WebDialogHelper(NativeWindow* window)