* 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
content::RenderFrameHost* render_frame_host_;
  content::FileChooserParams::Mode mode_;
};
file_dialog::Filters GetFileTypesFromAcceptType(
    const std::vector<base::string16>& accept_types) {
  file_dialog::Filters filters;
  if (accept_types.empty())
    return filters;
  std::vector<base::FilePath::StringType> extensions;
  int valid_type_count = 0;
  std::string description;
  for (const auto& accept_type : accept_types) {
    std::string ascii_type = base::UTF16ToASCII(accept_type);
    auto old_extension_size = extensions.size();
    if (ascii_type[0] == '.') {
      
      
      base::FilePath::StringType extension(ascii_type.begin(),
                                           ascii_type.end());
      
      extensions.push_back(extension.substr(1));
    } else {
      if (ascii_type == "image/*")
        description = "Image Files";
      else if (ascii_type == "audio/*")
        description = "Audio Files";
      else if (ascii_type == "video/*")
        description = "Video Files";
      
      net::GetExtensionsForMimeType(ascii_type, &extensions);