Fixed regression in file browser.

Fixed regression in file browser.

The GUI file browser was the only place in the code we were using new
and delete[] instead of HeapAlloc() and HeapFree().
The previous commit overlooked that fact and introduced a compiler error
by assuming that lpstrFile was a buffer of a known size instead of being
allocated with new.
Fix the regression and use HeapAlloc()/HeapFree() for consistency.