* 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
}
    if (extensions.size() > old_extension_size)
      valid_type_count++;
  }
  
  if (extensions.empty())
    return filters;
  filters.push_back(file_dialog::Filter());
  if (valid_type_count > 1 ||
      (valid_type_count == 1 && description.empty() && extensions.size() > 1))
    description = "Custom Files";
  filters[0].first = description;
  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
  }
  
  filters.push_back(file_dialog::Filter());
  filters.back().first = "All Files";
  filters.back().second.push_back("*");
  return filters;
}
}  
namespace atom {
WebDialogHelper::WebDialogHelper(NativeWindow* window, bool offscreen)
    : window_(window), offscreen_(offscreen), weak_factory_(this) {}
WebDialogHelper::~WebDialogHelper() {}