For WIndows :
When you download the file with Internet Explorer, it has a .zip extension,
change that to .ipsw. Don’t unzip it, just change the extension (rename the .zip file in .ipsw).
For Mac :
On a Mac if you read “xxxx.ipsw.zip” delete .zip.If you don’t want to change the file type to .ipsw, just use different browser such Firefox or Safari from Apple.