It seems that you'll just need to type in a new file name instead of picking one. When I pick an existing file I get an error "file with that name already exists". I tried with .txt and with no extension and both worked fine.
This looks like it. I tried this a little after I posted and got it to work.
This is what I used in VIC 2. For example, if my datastore name was Data I had to type the following into the output file text box: