You create the listbox inside a loop. That means every loop iteration, you make a listbox on your selected position with only the i'th index of filename. You need to set up your entire list of items first (e.g. inside the loop), then create your listbox after the loop. For example:
yourcell={sFiles(1).FileName};
for i=2:1:length(sFiles)
yourcell=[yourcell,{sFiles(i).FileName}];
end
res=uicontrol('Style', 'listbox','Position',[50 200 1000 200],...
'string',yourcell,'max',10,'min',1);
