Pointing to a filepath that does not have an extension just downloads the file

If users create an nbgitpuller link to a file that has no extension (e.g. `myfile`) then the "redirect" step will trigger the browser to simply download the file, and remain stuck on the nbgitpuller loading page. We should ensure that nbgitpuller at least always tries to open the file with the text editor if it isn't `ipynb`.