Well this is defintely strange if your a noob.
On the DreamHost web servers, the filename extension is used to determine what "type" to tell the browser so that the brower has an idea what to do with the content.
By default the type for ".html" files is text/html. It sounds as if the web server for your site thinks it should be text/plain.
There are a couple of places that influence what type the web server tells the browser. Obviously there is a master configuration file (ie server configuration) but I doubt the problem is there. You can place .htaccess files ("per-directory configuration") in your web directory that changes the types - so I would check to see if you have any of those. And last there could be a handler program involved somehow but I doubt that as well.
Try going to the DreamHost Web Panel and chose "Manage Domains" to "Edit" the domain and submit the form - it will update the server configuration.
Also, go to http://web-sniffer.net/ and submit your URI - look in the "HTTP Response Header" section for the Content-Type value. Does it say text/plain or text/html there?