1. Create new process OR dmn file
2. Model something (star -> end)
3. Input name without .bpmn suffix or .dmn suffix and Save (save passes)
4. Use Recent to open it
There will be error about unsupported file.

WORKAROUND:
When saving, add corresponding file suffix - .bpmn or .dmn into file name.

I expected the HUB will add the suffix for me, for the reason this is tooling for BPMN and DMN.