You could try it with 'tokenat'.
tokenat(src, index, separator)
Return the Nth token from src, splitting the string by separator. For example, tokenat("apples|oranges|bananas", 1, "|") returns oranges.
Something like:
File Choooser > On Changed > Array > Set Value at 0 to tokenat(FileChooser.FileNameAt(0),0,".")