This is just a starter question for you and may be useful whilst we wait for an expert.
Did you define your site under site management?
Doh! I re read your post and can see that you have.
I still wonder if there might be something in there that is a bit awry?
This sounds like you have a corrupted user config folder and DW is not recognizing the file types.
First back up all your site definitions, then uninstall DW, delete your user configuration folder and then reinstall DW and any extensions you may have.
For details on locating your user config folder see - http://forums.adobe.com/thread/494812.
Thank you for your replies Martin and PZ. I deleted the Dreamweaver 5.5 user config folder + Dreamweaver Registry entries before reinstalling Dreamweaver CS 5.5. When I first started working on the website, I was impressed to find that content I added to the site's root and images folders (with Dreamweaver closed) was present in Local Files upon opening Dreamweaver 5.5. No matter what I do in Manage Sites, the website's folder in Local Files is empty.