The files that you attach to an email become locked until the instance of Powershell you are running has exited completely. So if you run a script through Powershell ISE that attaches a file to an email, that file will remain locked until you exit Powershell ISE.

If the file is locked by Powershell, you will get an error/warning message similar to the following if you try to modify it in any way;

The process cannot access the file 'c:\filename.txt' because it is being used by another process

By using the following command, you can ensure that Powershell 'disposes' of the email message once it has been sent and does not continue to 'lock' any files you attach and send via email;

$mailmessage.dispose()

Note: this is assuming that $MailMessage = New-Object system.net.mail.mailmessage