(I see this is Wikimedia Commons and I haven't been able to test it elsewhere)
There seems to be a restriction not allowing new users to overwriting existing
files by uploading new ones with the same name (according to this:
[http://commons.wikimedia.org/wiki/Commons:Help_desk#Revising_images]). This is
all fine. The problem is that it also prevents new users from overwriting files
that they _themself_ have created. Obviously, this does not help to stop
vandals, it only annoys new users (especially since the error message does not
state the correct reason).