Hi there, starting in Tableau 10.0, the copy image functionality on Windows only copies to BMP format. This is a deliberate choice based on some technology constraints with the new Tableau font. You can still export to EMF if you use the Export Image functionality by going to the menu and choosing Export > Image. You will see EMF as an option there.
You can find more details in this help article:
Rubaiyat Khan, Senior Product Manager, Tableau
For those of us that do this type of high resolution cut and paste extensively for analytical summaries, it cumulatively injects quick a bit of non-value added time to the process. I sure hope future revisions restore this functionality.
If you agree that this functionality should be restored, consider "up-voting" the idea in the Tableau idea forum (link below).