I would call it a bug, Mirc is not doing what it used to or what the help says its ment to. Resulting in files that can be deleted. The Option to use sepreate folders is all well and good, BUT isnt the solution to a "bug" in the dcc resume system.
I also well note becuase i didnt bother to in the first message, that the filename returned isnt FILE.EXT or the filename being sent (as its stated to do from 6.0? 6.1?) its just "" this also appears to cause some problems for non mirc clients sending files, but im assuming that these two things maybe connected, possably for some reason the sending filename isnt being passed to the resume routines correctly, thus the filename "" is checked and said to be zero bytes long, thus the resume is requested from zero. If that is the problem, then simply correcting the filename being passed to the resume routine, well eliminate the resume from zero problem.