There is a great explanation of the correct way to "serve up" XHTML with the correct mime type here:
Serving up XHTML with the correct MIME type
A PHP code sample is illustrated showing how you might deliver the correct header appropriate to the host server.
Useful Reference:
Mime Types List - shows you a list of common MIME types and their corresponding file extensions.
The important thing to remember is the
purpose of the Content-type meta tag. By using the Content-type (mime type), you are effectively telling a user agent (Web browser, PDA, etc) what sort of content is contained within your document. This can be read from the document's header without the need to explore the body of the file.