We used to have a two-step process for answering questions. On the first page the user indicated how he would answer: either by typing a response or uploading a file. On the next page either a textarea or file input was presented. Instead I plan to present a single page where the user can do either of those things, but not both. What is the most user-friendly way to do this?
You could create 2 radiobuttons. So that the user can make the choice whether he wants to add text or upload a file.
If you do this it is very important to also check it at the backend code (php / asp / whatever) since it would be trivial to circumvent this (by disabling JS or by changing the DOM).