How to output details with uploaded file?

Hi,

Can someone give me a hint how to output extra details of the file after it has been uploaded?
While uploading, the widget gives details on filesize. I would like to show the filesize and other stuff when the file has finished uploading (now we just get the file link).

Can I achieve this in a custom module? Or does it need to be done in theme functions?

Thanks for any help!