Fix a bug that <input type=file multiple> doesn't accept many files.
The old code restricted the total characters of selected files to MAX_PATH (256).
It is insufficient for multiple files. The new code uses UNICODE_STRING_MAX_CHARS
(32767), which is the maximum string length of Win32 API.
Firefox uses 4096.
TEST=No automated test because it's very hard to test the behavior of
Windows' file open dialog.
Manual test: Make two files on a local disk. The length of their names
should be 200 characters. Visit a page with <input type=file multiple>,
click the file upload control, and select these two files. The file
upload control should show "2 files".