what happened behind the GMail screen when attach a file?

[athos]

hi guys,

have you ever noticed, that in gmail composing email page, when you
choose to attach a file, first the ful path of your local file is shown
in bold, then, after a few second, seems the file is uploaded to server
and only the name is shown and not bold now, but italic.

anybody knows what happened behind the scene? seems it's quite useful,
especially if there is no limitation about the uploading, for, nowadays
the html file element for uploading always has a size limitation of
about 4M (configurable at server side, but there is always a
limitation).

thanks!

yours,
athos