If I were in the OPs situation then how I would react would depend on two things. How the higher ups at the company view me already and how much I wanted to progress. I've recently been given a promotion (at the expense of someone else getting a demotion) purely because I don't have a "that's not my job" mentality. At work you should be flexible enough to do what's best for your company but that doesn't necessarily mean any work you do that's not in your job description has to be extra work. If you're taking odd work like this on then just point out to your manager that it'll have to be at the expense of something else and ask what they'd like you to focus on.