Instead of having multiples of the same files in different formats (png, pdf, psd), you can nest all of the different formats together. It is most commonly used when working with logos. Nesting files together avoids visual clutter in your library.
This tool may also be used to nest the .xmp file with your raw files.
Locate the file you wish to add additional files and go to the Details view.
Click the Ellipse in left corner, then Add Attached File.
When you are back in the library view and you hover over the file now, the number of nested files will be noted for you. All additional files will be available in the Download section.
You can also review and remove nested files in the Details tab. The nested files are listed under Additional Files and can be removed by clicking the Trash icon to the right.