I have created a program to lock files and by default when a file is locked the file changes from being a folder to a lock. So what I want to do is to change the lock to my own image, just like how VLC do, it changes all the MP3 files to VLC icon if VLC is installed as a default media player so I want to do the similar thing too.
@ddanbe, no, I didn't mean that. What I meant was that mmmmm let's make Visual Studio an example. The extension (.sln) is not recognized within Windows to which application can open it if you haven't installed Visual Studio on your computer, but once you have installed it, then the (.sln) will change to Visual Studio. (Not sure if you understand), another example is (.docx) which was introduced in Microsoft Word 2010 I think because before that it was (.doc) so the (.docx) maybe recognized by MS Word 2003 but won't be supported or opened till you install MS-Word 2010 or higher so with this extension file in your computer but not having MS-Word installed its will be of unknown.
One more last example is (.pdf) if you have this file but don't have Adobe Reader installed you won't be able to open this file. So coming back to my question, how to make my file extension have my own icon, like (.pdf) will have the Adobe Reader icon, (.txt) will have text icon, (.doc) will have Microsoft Word icon, (.xls) will have Microsoft Excel icon if Microsoft is installed?