The File Name Settings Tab is the second tab on the Web site Settings window. It can be accessed by clicking on the Web site Settings button on the Other tab, or by the Settings menu.
This tab lets you choose the default file extension for your pages, whether filenames will be converted to lowercase, what character will replace spaces and illegal characters in filenames, and the name of the css file that is used.
File Extension - Use the drop down list to select the default extension for the pages in the site you are currently working on. Making changes to this setting will not have any impact on any other sites you have created.
Convert Filenames to Lowercase - If you would like all your filenames to default to lowercase put a check in this box. If you leave this box unchecked the files will be named exactly as you enter them on the Web pages Info tab. Many webmasters choose to use lowercase for all filenames because it makes it easier when publishing urls. Some servers, such as Unix servers, are case sensitive which means that if someone enters www.yourdomain.com/new.html when the url was actually www.yourdomain.com/NEW.html they would get an error. To save problems down the line it’s usually wise to go all lowercase for filenames.
Replaces spaces and invalid characters with - This allows you to choose a character to replace any spaces that appear in filenames. For example is you had a file called "the latest news.html" and you chose to have spaces replaced with an underscore (i.e. _) the filename would automatically be replaced with "the_latest_news.html” and if you chose hyphens it would become “the-latest-news.html”. Some people prefer underscores others prefer hyphens – the choice is yours.
Layout CSS filename - By default the css file for your site will be named after the title of your site. If you wish to change it you can do so by entering the new name in this box. Please remember to include the .css or some other appropriate extension.