A file with the ASPX extension is an ASP.NET source file. While it is best to let a computer decide which program is best to open the file, any web browser, such as Internet Explorer or Mozilla Firefox, can open a file with an ASPX extension.
The actual code in the ASPX file is processed by the web server and can be coded in any program that codes in ASP.NET, such as Microsoft Visual Web Developer. It can also be opened using Adobe Dreamweaver.
When a website is experiencing problems, it sometimes provides ASPX files rather than the information intended. One way around that is for the user to give the file the extension name for the type of file expected. For example, if a user expects a PDF file and gets an ASPX file instead, he should simply change the ".aspx" extension to ".pdf".