Where do you think I should begin?
Begin by writing down a set of use cases that describe all the different ways you can envision for people to use file expiration. Also write down the risks (automatic destruction of evidence, security concerns, etc.).
Next, figure out how the feature should work. How will you keep track of the file expiration date? Is there any way to get an expired file back, or will it really be deleted? Would it be better to have the file system simply hide expired files and only delete them when the space is needed? Developing a detailed understanding of the feature's behavior now will save you a lot of time and effort later.
The next step is to design the feature. Creating some sort of design document will help you stay on track and remember what you're doing even when life intervenes at some point and you have to step away from the project for a few days or weeks. It'll also help you explain the project to others in the future. It doesn't have to be beautiful, but try to capture as many of the decisions that you've made and the reasons behind them as you can -- what's obvious to you today won't be in a few months.
Once you have a design, you can get down to implementing it. Depending on your style, that might mean that you first write a set of tests, or jump right in and start work on the feature itself... but that really gets away from the scope of your question.