Change upload file URL relative to absolute

Hello,
I am using FCKeditor on an email system.
When users want to build a link in their email to an uploaded file (using the link tool), the resultant URL is relative
eg /userfiles/uploadedfile.pdf

However, since it will be transmitted in an email, the URL needs to be absolute
eg http://www.mysite.com/userfiles/uploadedfile.pdf

Does anyone know what file to change, and what code to change to set this automatically?

Thanks in advance