General web rules prohibit you from writing code in the browser that runs apps on the client PC. (Yeah, plenty of viruses get around it, but you want to write an app, not a virus.) If I enter an address in my browser that happens to end with ".doc" my browser makes a temp copy of the file on disk and starts up
Word. I can edit the thing to my heart's content, but I have to save it to a new name or the browser will delete the temp file.
If you want to edit a file that resides on the server and update it there, things get much harder. If you're big into Microsoft tools, WebDav might be the answer. Search MS for webdav and see what pops up.