In a recent update of iA Writer, the application changed the default file format from MD to TXT. Since I use iA Writer to write in markdown, as I assume most of you do, this change came as a bit of a nuissance to me. Furthermore, there is no options in settings to configure. The only option is to start writing your file in .txt format, and later rename it to .md.
So I have made a tweak called Default MD in iA Writer, available on my repo. It makes the default file format .md instead of .txt. Please make sure you have opened the iA Writer application and created a document in it at least once before installing the package.