Renaming multiple files with the same extension using PowerShell

I would like to rename just the csv files in my H:/DMU folder. I tried the following code:

 Dir -filter *.csv | %{Rename-Item $_ -NewName ("fn*.csv" -f $nr++)}

I encountered the following error for every file I tried to convert. PowerShell Error Code

I know how to rename all the files in this folder to include the prefix “fn”

Get-ChildItem -Exclude "fn*" |rename-item -NewName {"fn" + $_.Name

, but I just don’t know how to rename those of a particular extension.