For a ready example upload anything to
http://rapidshare.de
on the successful upload page just try entering anything into the email address field.
The other extensions don't seem likely culprits
Add n edit cookies
bookmark backup
cookie culler
remove it permanently
session saver
smart search
tab mix
user agent switcher
platypus
the cleaner
image shack
fillfilesuploadform
greasemoneky
AdBlock Plus 0.5.8+ dev
But then it might just be me

thanks for your great extension!!