How to enable the shared clipboard in VirtualBox

Just for the record, one year after the OP's question ;-). But well, this is an interesting point...

1. No, VirtualBox shared clipboard does not (read: never did) allow to share objects, files or images between guests/host.

2. Yes synergy will happily share images (but not files AFAIK) across heterogeneous systems.

3. However, if you run VirtualBox in your main workspace or don't have a second monitor, the situation will probably be confusing (I actually never tried to run synergy in a guest, just imagining) because there will be two conflicting [virtual] mouse:

   • the synergy captive mouse enters in action only if you [attempt to] "cross" the boundary of your synergy server monitor,
   • so the virtualBox guest addition shared clipboard will have the last say, and we are back to square one.

If someone ever tries this hybrid, please publish the outcome. Or any better solution for that matter.

If it's a file(s) or folder you're trying to copy on the shared clipboard, I don't think that type of clipboard data is supported. When you copy a file, Windows doesn't write the entire file contents to the clipboard, it just writes a reference to the file. So it would make sense that you couldn't paste a file into a guest from the host's clipboard, since the guest doesn't know where that reference is pointing to. Try copying some plain text between the guest and host - if everything's set up correctly you should have no problem doing that.