Cannot save file back to web share folder

I have a C# web application that creates an RTF file and saves it to another web share on the same PC (IIS Virtual Directory) C:\Letters

When I open the file it opens up in read only mode. I want the user to be able to edit the RTF and save it back to the same web share location rather than do a 'Save As' - which is able to overrite that file.

The Virtual Directory has full read/write/browse permissions.

i.e. I can do http://localhost/Letters/myLetter.rtf in a web browser and it will bring up the file in ms word - BUT in read only mode!

I want to also be able to save back the file after the user clicks save in word.

Any ideas of anything else I need to set?

Instead of opening via http, redirect the user straight to the RTF file with its UNC path.