Use question fields to automatically rename uploaded file in Qualtrics form?

Hi everyone,
I've been digging through forums, stack overflow, etc. but I can't figure this out.
I have a Qualtrics form that asks for 3 things: 1) First name, 2) Last name, 3) a file upload (image file).
Can someone help me with the Javascript needed to use the fields in questions 1 and 2 to automatically rename the file with the following conventions?
Lastname_Firstname_[a word of my choosing for all file uploads].
Any help would be greatly appreciated!