Plug-in Files Extensions - What are plug-in files and how do I open a plug-in file?

What are plug-in files and how do I open a plug-in file?

File Type Category File Format
.dpm Plug-in Files Pro Tools Plugin File
.ecf Plug-in Files Outlook Add-in
.vnt Plug-in Files Mobile Phone vNote File
.xpi Plug-in Files Mozilla Installer Package