Version of VS Code

1.29.1

Your operating system

Ubuntu MATE 16.04

List of extensions that you have installed:

None

Reproducible steps (1... 2... 3...) that cause the issue

1. File->Preferences->Extensions
2. Click ellipsis button (...)
3. Click "Install from VSIX..."
4. Try to choose multiple .vsix files, e.g. with Shift+click or Ctrl+click. It is not possible.

What you expected to see, versus what you actually saw:

I expect to be able to choose multiple .vsix files and install them all simultaneously. Instead I had to click "Install from VSIX..." once for each extension.