Adobe Dreamweaver will set itself as the default editor for the following file types:

Active Server Pages (asp)
Java Server Pages (jsp) [not automatically selected, did not select manually]
ColdFusion (cfm,cfc)
ASP.NET (aspx, ascx) [not automatically selected, did not select manually]
PHP (php, php3, php4)
JavaScript (js)
Cascading Style Sheet (css)
Extensible Markup Language (xml)
XSL Style Sheets (xsl, xslt)