The .APPXUPLOAD file extension was introduced by Microsoft along with their Windows 8 operating system. This format replaces the .XAP files which were a part of the Windows Phone operating system , also commonly referred to as Windows Phone 7.

Both .APPXUPLOAD and .XAP files are App package formats used by developers for packaging applications and uploading them to the Windows Store. The .APPXUPLOAD files contain the .APPX package as well as the symbol debugging information needed for the app crash reports used by developers for bug fixing.

This information is stored in .PDB (program database) files, which are created automatically during the App packaging process. Besides these two files, .APPXUPLOAD packages also contain a .APPXSYM compressed file which stores the public symbols of the application.

How to open or create an .APPXUPLOAD file

The program used for creating and opening .APPXUPLOAD files is Microsoft Visual Studio 2012. After successfully developing an application, before submitting it to Windows Store developers need to build it in Release mode and create the .APPXUPLOAD package.

This is easily done through the "Create App Packages" wizard included in Microsoft Visual Studio which can be accessed from the Project menu by selecting Store and Create App Packages.