While looking through a folder of workspaces with a text editor, I noticed that one workspace fmw file was not in the usual XML-like format. It was instead a JSON file with a leading header line:
# application/x-fme-json-workspace
The last build/saved metadata states "FME(R) 2020.0.2.1 (20200511 - Build 20238 - WIN64)" - however when I create a new workspace with this version, it is saved in the XML format. I tried a variety of FME Desktop versions from 2019 to the most recent 2022, none save to JSON, but all are able to open the JSON fmw.
This question is in part simply out of curiosity, and also because I have made a small Python script that iterates over a folder to gather some metadata that the WorkspaceReader does not publish (luckily the attributes I'm looking for are in the same "path" in both formats).