Martin, I cleared the flag bit because filename was changed in-place, to
mark that filename does not need further processing. This was primarily
compatibility concern, to accommodate for situations where users try to
do such decoding in their own code (this way flag won't be there, so
their code won't trigger). Without clearing the flag bit, calling
_decodeFilenameFlags second time will fail, as well as any similar user
code.

I suggest that if users want to know if filename is unicode, they should
check that filename is of class unicode.