ShowOpenDialog on windows needs to use passed in file types and multiple selection

[tvoliter]

See brackets_extensions.cpp::ExecuteShowOpenDialog

Note: would also be good to support all file types

[njx]

Marking to user story. There's no functionality currently in the product that needs this.

[pthiess]

Added to Trello, keep issue open until team decided on tool.

[pthiess]

Keeping this one open since conradz is working on it, this comment should also remind us that - once done we change the existing user story to Mac only before closing this issue.

[pthiess]

Moved to https://trello.com/c/430aXkpq
closing herein.