fill a combobox with a sorted files in dir c:\,ydir\

i have in a dir_

1.txt
2.txt
55.txt
20.txt
9.txt
11.txt
...
6.txt
123.txt

how to fill a combobox with a sorted list of files, similar?

1.txt
2.txt
6.txt
9.txt
11.txt
20.txt
55.txt
123.txt