Question

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.

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?

Answer 1

Here are some generic best practices around file associations. One relevant part …

Do Not Use Short File Name Extensions

Long file name extensions offer the following advantages:

The limited length of short extensions make them prone to extension
collisions. An extension collision occurs when the same extension is
used to classify multiple file types. Using long extensions
significantly decreases the chances of a collision. Short file names
tend to be somewhat cryptic. Long extensions tend to be more
meaningful because additional information can be embedded in the
extension.

For Metro style apps, the only limit I could find is in this document …

Important note: Here is an alphabetic list of file type names that
you cannot enter into the package manifest because they are either
reserved or forbidden: Accountpicture-ms, Appx, application,
Appref-ms, Bat, Cer, Chm, Cmd, Com, Cpl, crt, dll, drv, Exe, fon,
gadget, Hlp, Hta, Inf ,Ins, jse, lnk, Msi, Msp, ocx, pif, Ps1, Reg,
Scf, Scr, Shb, Shs, Sys, ttf, url, Vbe, Vbs, Ws, Wsc, Wsf, Wsh.