Can I tell Visual Studio how treat a file with a custom file extension?

In Visual Studio go to Tools > Options > Text Editor > File Extension

There you can add specific extensions and choose which 'Editor' Visual Studio uses for that extension.

There is a registry hack that you can use. http://www.engagesoftware.com/Blog/EntryId/117/Editing-SqlDataProvider-scripts-within-Visual-Studio.aspx

Basically, you look at HKLM:\SOFTWARE\Wow6432Node\Microsoft\VisualStudio\9.0\Languages\File Extensions\.css (changing the version number based on your version of VS, and removing Wow6432Node if you aren't on a 64-bit OS) and make a copy of that, calling it .my-custom-extension

UPDATE According to a similar Stack Overflow question, for some extensions (i.e. CSS), there's another step to be taken. It says to take the GUID from the extension that you're copying, and find its entry in HKLM:\SOFTWARE\Wow6432Node\Microsoft\VisualStudio\9.0\Editors and add a DWORD value in its Extensions key like the existing css value. That question also points to an extension in the VS gallery that does that. However, even after doing that, I'm still unable to get syntax highlighting, myself...