Rename-Item : Cannot create a file when that file already exists - it doesn't

Although it seems these are not complete files (fragments of a larger download perhaps), the main issue with your code is that you need to use a scriptblock on the -NewName parameter in order to use the $_ automatic variable as sirtao already showed in his answer.

But also, beware that piping files directly from Get-ChildItem into the Rename-Item cmdlet may result in trying to rename files that already have been renamed.

To prevent that from happening, enclose the Get-ChildItem part of the code in brackets so it will enumerate the files to fetch completely before sending them through the pipe to rename them.

(Get-ChildItem -Filter '*.part-frag*' -File) | Rename-Item -NewName { '{0}.png' -f $_.Name } 

or first capture the enumerated files in a variable and use that to rename the files

$fragFiles = Get-ChildItem -Filter '*.part-frag*' -File $fragFiles | Rename-Item -NewName { '{0}.png' -f $_.Name } 

It does exist: using "$($_.Name).png" you end up creating a file named .png because $_ is not accessible the way you wrote it and thus defaults to null which gets converted to a empty string and then hained to .png.

Solution: encapsulate it in a scriptblock: Get-ChildItem *frag* | Rename-Item -NewName { "$($_.Name).png" }

-whatif is your friend:

echo hi > frag Get-ChildItem *frag* | Rename-Item -NewName "$($_.Name).png" -whatif What if: Performing the operation "Rename File" on target "Item: C:\Users\js\foo\frag Destination: C:\Users\js\foo\.png". 

With scriptblock and slight changes. Usually if you can pipe to a parameter like -NewName, you can use a scriptblock with it.

(Get-ChildItem *frag*) | Rename-Item -NewName { $_.Name + '.png' } -whatif What if: Performing the operation "Rename File" on target "Item: C:\Users\js\foo\frag Destination: C:\Users\js\foo\frag.png".