DAT File:
The DAT file is nothing but a data file. DAT file is a generic data file generated by a specific application and more over it can be accessed only by that application and not by other application. Such DAT files consists of all supportive information which are very badly need to run that application. In its absence the compiler or the control fails to run that application.

file extension dat may contain the data in the form of graphic or text or binary or any other format, consisting of object code. Generally text-based DAT files can be viewed using some text editors like notepad, WordPad, etc. If we open the binary format DAT file in any text editors it will show some symbols, this is why because such text editors cannot encode these types of binary data.
The information may be either the date and time of last modification, last accessing, meta data–consisting of position and property with full description for any components used in the application, etc.
Each software, it may be either system software or application software create, open, or reference only its own and unique DAT files. Some instances include: Microsoft Visual Studio, Adobe Photoshop, Nero ShowTime, Nullsoft Winamp, etc.

Extension of ASPX:
aspx is an acronym, derived from the term Active Server Page. These types of files are created or generated by the server side scripting language, ActiveX. It was developed by Microsoft Corporation and was launched during the year 2002. Actually ASP is the predecessor of ASP .NET, which is used to build dynamic web based application in an easy manner.
ASP .NET server controls brings out an HTML-like style of declarative programming that allows us to build dynamic web applications or web sites with far less code than with the predecessor ASP or technologies like PHP or JSP. Best of all, ASP .NET pages work in all browsers – including Firefox, Opera, Safari, and Internet Explorer. ASP .NET is built upon the Common Language Runtime (CLR) as it supported automatic garbage collection, runtime error checking, object oriented programming concept, allowing programmers to write ASP .NET code using any .NET compatible language like C,C++,C# etc.
The first prototype was called “XSP”, Scott Guthrie, one of the developers of the prototype, explained in a 2007 interview that, “People would always ask what the X stood for. Initially and really it didn’t mean anything. XML,XSLT started with that. Better looks cool to start with an X, so that’s what we originally named file extension aspx (ASPX).”
With the migration to the Common Language Runtime, XSP was re-structured in C# and renamed to ASP+, as by this point the new platform was seen as being the successor to Active Server Pages, and the purpose was to provide an easy migrating path for ASP developers. Once the “.NET” branding was decided on in the tail period of 2000, it was decided to rename ASP+ to ASP .NET.