Hi LA,
Your version is 9.1.0000.10733 while mine is 9.1.0.10129,
I still didn't find File in last of Date Type drop down list.
As you mentioned, your environment is in a new Trial,
could you share me how did you register a new trial instance?
As doc in my first response said: This topic is pre-release documentation and is subject to change.
So it might be a new feature under testing, we wouldn't need this new data type field if everything works well currently.
And due to limitted announcement of it,
I suggest you could open a support ticket for detailed anwser about the field.
https://admin.dynamics.com/?referer=mbssupport
Regards,
Clofly