How to change default extension to .md and save to a folder name of my choosing

[Tony P]

I'm thinking of use this to edit my blog online by editing the markdown files but I need the .md extension, not .txt. I tried changing all "txt" in the main file (index.php) but it didn't work. I also need to save to another folder name than "pages". What do I have to search and replace in index.php to do this?