javascript simualte choosing a file

javascript simualte choosing a file

I am trying to make a java script that i can just enter in my browser and
will automatically choose the file.
I know i can fill forms and such by using a script like
document.getElementById('user_name').value='admin';return false;
But im not sure how to do it for choosing a file that i predefined for
example in c:/file.txt . How would that work for something like the html
below?
<input name="Configration_File" type="file" id="Configration_File">