I contacted Google and got it straightened out. The data in that file was the videos that had been saved in Google Drive, then marked Offline for viewing when not connected to the Internet. That loaded them back into my phone into this file in the Google Drive app.
The fix, deleting the contents of that file without disturbing the files in Google Drive, was
Settings -> Accounts -> (my account) -> Drive and uncheck the sync box. Then
Settings -> Apps -> Drive -> Clear data
That deleted the remnants of the old offline videos (which I had already taken off offline status in Drive) and freed up the storage space in my phone, 2.46GB worth.