Manually empty an upload field via script?

Manually empty an upload field via script?

I have a file upload field that is hidden or shown depending on whether a radio button is checked as "yes" or "no". That is simple enough to do:

if (Radio.contains("Yes"))
{
    show File_upload;                  
}
else
{
    hide File_upload;          
}

But, along with hiding the field, I also want to flush it of any files the user may have uploaded, since he could possibly be doing that by accident, and after realizing it, chooses not to click the "[remove]" option next to the field, and instead checks the Radio Button as "No", thus hiding the upload field, and leaving the file there.

I have tried doing File_upload = ""; and File_upload = null;. Both of them compiled fine, but did nothing. How do I go about doing this?