There is only one file type that uses the .NZB extension:
NewzBin Usenet Index File
An NZB file is an XML file used to retrieve posts from Usenet servers. It contains summary information about a Usenet post, including the post's ID, poster name, posting groups, file names, and file segments. NZB files are similar to .TORRENT files, in that they are files used to download other files.
NZB files are typically not meant to be opened. Instead, they are meant to be imported into a Usenet client and used to download a Usenet post. For example, in the SABnzbd client (cross-platform), you can import an NZB file by clicking thebutton found in the Add File section of the app's Home screen.
If you want to view the XML-formatted text that an NZB file contains, you can open the file in any text editor.
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Updated: May 26, 2021