The metadata_never_index extension is associated with the Macintosh Operating Systems. These files are hidden by default on Apple devices and they're used to prevent applications such as Spotlight from indexing certain files, folders or partitions, hence the "never index" part in the name of the format.
Programs like Spotlight dispose of advanced searching methods on Apple devices which are based on file indexing. However, indexing can sometimes be redundant, especially when it comes to folders or partitions containing a separate operating system such as Windows.
Metadata_never_index files are also stored on flash drives, iPods and other portable storage devices to prevent indexing every time the device is connected to the computer. They can be found at the root of the volume. To enable Spotlight to index a certain storage device or folder users just have to remove the metadata_never_index file.
Disabling the indexing of a folder or volume that allows it by default can also be done by creating a new file with the metadata_never_index extension and placing it at the root of the volume. For folders it must be placed at the top of the hierarchy, not in a subfolder.
How to open metadata_never_index Files
Metadata_never_index files are system files that are not meant to be opened by the user.