I was looking for the gnome-exe-thumbnailer (or similar) package in Fedora 20 (which surprinsingly wasn't in the repos) but I found tumbler-extras, which I installed and almost immediately uninstalled; then Thunar was unable to generate new thumbnails.
I've made everything described here: Why is Thunar not creating and showing thumbnails of images?
I've also made a rollback to just before tumbler-extras was installed, but anything seems to be solving this issue.
I've just found out that this issue is caused by selinux. I've already filed a bug #1058064 on redhat's bugzilla.