AFAIK, "filename" is an opaque string according to TFTP spec (or at least it does not contradict this statement). So the client may pass whatever separator he feels like. Seems, that they are actually quite likely to use backslash as a separator (WinPE PXE boot comes to mind).

Nevertheless, I don't think, that this should stop us from merging this. If the need arises to translate backslash-separated paths into unix-style paths, it should be implemented separately as a new feature.