OS による制限

MSDN GetOpenFileName Function より

Note, when selecting multiple files, the total character limit
for the file names depends on the operating system and the
version of the function.


Windows 95/98/Me: (only ANSI is supported) no restriction
Microsoft Windows NT4 and earlier: 32k limit
Windows 2000/XP: (ANSI) 32k limit, (Unicode) no restriction


いまどきなら

Windows 2000/XP: (ANSI) 32k limit

これにひっかかりそう