Following up your example with filenames, you often see song titles "Jon & James", but I've never seen "Jon& James"-- the latter might exist, but the former is equally problematic. So as I said, if a user wanted to match files they'd have to know about & and escape it regardless of whether it matched a "whole" word or a "partial" word.
Without testing, I don't think you'd have to escape "Jon & James" because & as a word wildcard would still match & in the text (as a space deliminated "word").
That said, I do think that the functionality being extended would be a good thing. & should work in all situations:
a& b - Works for something like "apple b"
&a b - Works for something like "banana b"
a&b - Works for something like "arab"
a & b - Works for something like "a plus b"
a & - Works for something like "a plus"
& a - Works for something like "plus a"
The only issue is "a & b" due to the bitwise operation. I'm not sure how that can be handled, but the rest should definitely work.
