gci c:file | select *

$ext = ".xlsx"
$desc = (Get-ItemProperty "Registry::HKEY_Classes_root$((Get-ItemProperty "Registry::HKEY_Classes_root$ext")."(default)")")."(default)"
$desc

Microsoft Excel-regneark   #Norwegian description

#You could define this inside Select-Object too, but it's a bit long so I extracted it first to clean up the code.
$extensiondesc = @{n="ExtensionDescription";e={ (Get-ItemProperty "Registry::HKEY_Classes_root$((Get-ItemProperty "Registry::HKEY_Classes_root$($_.Extension)")."(default)")")."(default)" }}

Get-ChildItem |
Select-Object Extension, $extensiondesc

Extension ExtensionDescription
--------- --------------------
.oxps     XPS Document
.lnk      Shortcut
.txt      Text Document

$ext = ".bmp"

$desc = (cmd /c assoc $ext).Split("=")[1]
$desc = (cmd /c assoc $desc).Split("=")[1]

Write-Host $desc