.VSIXMANIFEST File Extension
What is a VSIXMANIFEST file?
The Extension Manager in Visual Studio uses the data contained in the VSIXMANIFEST file to install the VSIX file on your machine.
The VSIXMANIFEST data appears in the following format:
<?xml version="1.0" encoding="utf-8"?>
NOTE: When using project templates in Visual Studio, the manifest is named source.extension.vsixmanifest but then appears as extension.vsixmanifest when built and residing in the VSIX file.