file open modes notes

options:
require read
require write
existing (implied by require read)
allow multiple (only valid for require read)
directory

valid combos:

single_read: read_only checked,
multi_read
single_write
single_read_write_optional
multi_read_write_optional
single_read_write_required
multi_read_write_required
dir_read
dir_write (is this even needed?)
dir_read_write_optional
dir_read_write_required

single: all
multi: read, read_write_optional, read_write_required

checkbox: blank, checked-gray, blank-gray
blank => any or directory 
checked-gray => existing