New trigger - 'For a selected file' ID - no dynamic content yet
Having stumbled across the recent addition of the 'For a selected file' trigger (because it broke all of the Flows I had using 'For a selected item' for document libraries), I'm now in the process of rectifying the broken Flows.
I noticed that the ID dynamic content for the new trigger doesn't appear to have been set up. In the meantime, I believe you can use the expression triggerBody()['entity']['FileId'] . (Note: I'm yet to test the expression, I copied it from another action, will update if it doesn't work).