Embedding metadata when downloading a file

Just posted an idea about embedding metadata in files we download with
firefox (http://mholakovsky.blogspot.com/2008/11/history-of-
documents.html).

The main idea is to keep track where we downloaded a file. At least
for image files (JPEG, TIFF) this is achievable by adding XMP (IPTC)
container with the entry "xmpMM:DerivedFrom" to the file. This feature
should be configurable so every user can choose to enable it.

In case one arguments to never touch a original file compare how
professional image applications are working. For example Adobe
Lightroom is not touching the image content but adding and changing
the metadata of the image file (XMP container, see
http://www.adobe.com/devnet/xmp/pdfs/XMPSpecificationPart2.pdf).

I think this would be an outstanding feature, e.g. you could search
with your desktop search engine which files orginates from a certain
website without manually tagging your files. It would also enable
something like a workflow for other applications, e.g. in a graphic
program you use an image from your hard disk and you look for more
images from the orginal resource. With that feature it would be easy
to derive.