what i would do in a situation like yours is to use the files anyways and add a disclaimer in your readme file saying:
"this addon includes files from 3rd party designers, it is not my intention to take credit for them in any way"
i don't think that would be wrong, since you're borrowing files that are freeware, just don't borrow from payware - that would be stealing
that's what i would do... *don't kill me....
