What product/components do you use and which version/fix level?

webMethods ActiveTransfer Server 10.7 with fix MAT_10.7_server_Fix 5

What are trying to achieve? Please describe in detail.

ActiveTransfer Server can rename files in a post-processing action (either dedicated rename action or renaming during copy/move action). Is it possible to use placeholders/variables in the target file name of the rename step? I don’t want to rename the file to a fixed name since that would immediately lead to a name clash. I’d like to use placeholders/variables in the target file name for e. g. the original file name, a time stamp, or a counter (which picks the first free number similar to conflict handling in Windows Explorer). Is this possible? I didn’t spot anything such in the documentation. Or is there another approach for renaming files without running into name conflicts?

Yes,
variables available at runtime are documented in the section Server Configuration Parameters and Variables - Server Variables in the Managing_File_Transfers_with_ActiveTransfer.
You can also have your own variables, e.g. as the result of an invoked IS Service.