Does Explorer support document versions?

Yes, the behavior of SLIM Companion Explorer is as follows:
a. document libraries without versioning: default behavior is to overwrite an existing document
b. document libraries with major versioning: default behavior is to add a new major version (i.e. same as using Upload through the standard browser interface)
c. document libraries with major / minor versioning: default behavior is to add a new minor version (i.e. same as using Upload through the standard browser interface)