There is only one file type that uses the .VMDK-CONVERTTMP extension:
VMWare Fusion Temporary File
File created by VMWare Fusion, a program used for running software virtual machines; saved within a .VMWAREVM package and contains temporary information for the virtual machine.
VMDK-CONVERTTMP files can be very large in size (hundreds of gigabytes) and therefore may consume large amounts of disk space. It is possible that you delete these files without corrupting your virtual machine. However, before performing any manual changes you should either 1) copy the virtual machine before deleting the VMDK-CONVERTTMP file to make sure it doesn't corrupt the VM, or 2) consult VMWare Fusion technical support.
Updated: March 24, 2013