When you create a new file, and change the type of file that you want to make, the file extension is placed in the file name box. This seems unnecessary, as the extension is automatically applied when you create the file anyway, and it makes it more difficult to place your cursor in the proper typing location.
If your goal was to swap file extensions because they swapped file types, then you should probably only do so if they have already manually typed in an incorrect file extension.
If possible, automatically setting the focus to the file name input, after selecting an alternate file type from the drop down, would seemingly be a more productive change to the system.
Numbered Steps to Reproduce Problem:
Open a DM environment
File > New..
Select an alternate file type from the drop down
Watch your blank file name box turn into .ext
On an unrelated note, I would recommend changing the default project settings to this:
I have reworded portions, as well as changed the settings themselves, to provide what I believe is a more desirable and understandable setup.
//These are simple defaults for your project.
world
fps = 60 // Update 60 times per second
icon_size = 32 // Mappable tiles are 32x32 pixels each
view = 8 // Show 8 tiles in each direction (17x17 view)
atom/movable
step_size = 2 // Make objects move 2 pixels per step