It's complicated -- the letter blames both, but the actual lawsuit is aimed against the user who did the uploading.
Our client contacted the Wikimedia Foundation in April 2009 to request that the images be removed but the Wikimedia Foundation has refused to do so leaving our client with no option but to commence legal proceedings against you personally through the UK Courts. The purpose of this message is to inform you of the nature of our clientâ(TM)s claims against you and to give you an opportunity to settle the claims before legal proceedings are commenced. ...
Our client remains willing to enter into a dialogue with the Wikimedia Foundation to discuss terms upon which low-resolution images of paintings in its collection can be made available on the Wikipedia website and our client will continue to write to the Wikimedia Foundation with requests for discussion. However, to date, the Wikimedia Foundation has ignored our clientâ(TM)s attempts to negotiate this issue, preferring instead to take a more harsh approach that one would expect of a corporate entity.
And no, I have no idea what "a more harsh approach that (sic)
one would expect of a corporate entity" means.
Incidentally, the Wikimedia Foundation has a long-standing and consistent position
on claims like this:
WMF's position has always been that faithful reproductions of two-dimensional public domain works of art are public domain, and that claims to the contrary represent an assault on the very concept of a public domain ... WMF has made it clear that in the absence of even a strong legal complaint, we don't think it's a good idea to dignify such claims of copyright on public domain works.