title: Allowed file extensions for a Windows RT Application
body: <p>For my Windows Runtime Metro-Application written in c#, I want to use an own file format. For this file format I want to use an own file extension. In the package manifest I have declared the desired extension as a “File Type Association” and all works fine.</p>
  
  <p>My question is, if there are some rules or limitations for assigning/using a file extension. I clearly would not use a commonly used file extension such as “.exe”, “.cab”, or “.docx”. But are there any accessible guidelines, recommendations or rules, on how file extensions have to be built (3 chars, 4 chars, …) and which file extensions never should be used. Or even does exists a Microsoft-service where file-types have to be registered to be accepted later on in the app store?</p>