In most apps with file dialogs, Ctrl-K in the file name field deletes to the end of the line, but not in Sublime. When it makes the wrong assumption about the file extension, the natural tendency is to do what works everywhere else, and delete-to-end-of-line. But it does not work there.
UX Flow:
Create a new file
Cmd-S to Save
Give it a name (oops, can’t Ctrl-K here!)
I’ve tried this in a number of other apps, and it’s peculiar to Sublime that the File dialog does not work as expected.