luci-base: allow optional default value for file browser
luci-base: allow optional default value for file browser

With this change we could set optional value for the file browser.

Signed-off-by: Florian Eckert <ee3e4af9c48a69f5a5c47153eb4a777754bfbe6f@dev.tdt.de>

rogerpueyo/luci,rogerpueyo/luci,tobiaswaldvogel/luci,openwrt/luci,openwrt-es/openwrt-luci,Noltari/luci,nmav/luci,lbthomsen/openwrt-luci,hnyman/luci,artynet/luci,hnyman/luci,nmav/luci,tobiaswaldvogel/luci,openwrt-es/openwrt-luci,artynet/luci,openwrt-es/openwrt-luci,artynet/luci,hnyman/luci,hnyman/luci,openwrt/luci,nmav/luci,openwrt/luci,nmav/luci,openwrt-es/openwrt-luci,hnyman/luci,openwrt-es/openwrt-luci,lbthomsen/openwrt-luci,nmav/luci,Noltari/luci,openwrt-es/openwrt-luci,hnyman/luci,tobiaswaldvogel/luci,rogerpueyo/luci,artynet/luci,openwrt/luci,tobiaswaldvogel/luci,lbthomsen/openwrt-luci,lbthomsen/openwrt-luci,tobiaswaldvogel/luci,openwrt/luci,openwrt-es/openwrt-luci,Noltari/luci,nmav/luci,Noltari/luci,openwrt/luci,tobiaswaldvogel/luci,nmav/luci,lbthomsen/openwrt-luci,rogerpueyo/luci,Noltari/luci,artynet/luci,lbthomsen/openwrt-luci,lbthomsen/openwrt-luci,rogerpueyo/luci,openwrt-es/openwrt-luci,rogerpueyo/luci,openwrt/luci,openwrt/luci,nmav/luci,Noltari/luci,Noltari/luci,Noltari/luci,nmav/luci,hnyman/luci,artynet/luci,artynet/luci,hnyman/luci,lbthomsen/openwrt-luci,artynet/luci,tobiaswaldvogel/luci,Noltari/luci,tobiaswaldvogel/luci,rogerpueyo/luci,rogerpueyo/luci,artynet/luci