"on the attached" or "in the attached"

since the info is "inside" the file as you're going to open it, be it digital or a physical file, I'd go with "in".

The information is within the contents of the file, not placed on top of the file. Therefore, I side with using "in".

By contrast, using "on" feels like there is more to the sentence, like the information about the file... something. For example:

You can find the information on the attached file in the database.

French "sur" corresponds to English "on."

In either case, the prepositional object is keeping the modified from falling to Earth, or maybe farther... into oblivion. The thing that is under physically supports the thing that is "on."

Alternatively, strictly data-driven, I've never seen a usage in which "on the attached" seemed superior.