Sorry, but this version from today does not have .vhds available for creation/open
Until this is confirmed working, you have to enter the path like c:\temp\mydisk.vhds.
Once confirmed, in a future clonedisk version, the openfile dialog will then propose vhds next to vhd and vhdx (currently).
See this as a "hidden" beta feature for now : just for you
Currently, Clonedisk will detect, based on the extention, IF it needs to use VHDSET provider.
If the mydisk.vhds is created, then you got a winner - if not, clonedisk will moan about "missing provider".