I am trying to submit a new application in the Windows Phone Store. We decided to develop a Windows Phone 8 Silverlight application to support both WP 8.0 and WP 8.1.
When I upload the XAP file, I get the error that only .appx files can be uploaded. What does this mean?
.appx files are generated if you develop Windows Store apps (Windows 8/8.1) and as far as i know also Universal Apps.
it seems like you develop for Windows Phone! 8/8.1. What you get after compiling is a .xap-file.
When you log in to your developer account (https://dev.windows.com/en-us/dashboard) you can decide or should be asked if you want to go to the "Windows Phone Store" or the "Windows Store" dashboard. in your case you would need to choose "Windows Phone Store"