Well, it wouldn't be the File object you'd need to mock as such, it'd be whatever reader or stream you open to consume that file.
Having thought about it some more, it's probably not worth the effort. I'm sure it's possible, but the user's mapper or configure method code would need to be written in a way which allows the mock objects to be injected in the right places, and would require a lot of non-mrunit test fixture setup code that I doubt anyone would want to write.
Using temp files would definitely be a lot easier. Let's go with that.