There is only one file type that uses the .VSIX extension:
Visual Studio Extension
Software add-on created for Microsoft Visual Studio, a software development IDE for Windows; saved in a .ZIP format and contains files that extend the functionality of the Visual Studio software; stores a package description as well as binaries and other resources that comprise the extension.
Visual Studio extensions can be installed using the Extension Manager UI within the software.
NOTE: You can use the Visual Studio 2010 SDK to create VSIX extensions.
Updated: August 29, 2017