Cannot create files with arbitrary extensions

[firelizzard18]

Describe the bug
When I am using Arduino Pro IDE, which presumably is targeted at professionals, the create file action will not let me create a file with an extension that is not an approved extension.

To Reproduce
Steps to reproduce the behavior:

1. Set the window to advanced mode
2. Execute File: New File
3. Type in "README.md"

Expected behavior
I can create a file with whatever extension I want.

Actual behavior
".md is not a valid file extension"

[per1234]

Fixed by #263