I think that it's entirely possible to make the transition from film to IR. It might not be easy, and depending on your current salary you might have to take a pay cut. However, it's doable. Despite what a couple of other posters have said, late 20s is certainly not too old to be starting an MA in IR or similar degree. A lot of those programs have average ages of 25-27 for new students, so a few years older is certainly not a problem and depending on how you define late 20s, you might even be in the average range.
Certainly, you are not out of the range for most entry level jobs with an MA. The average age for new FSOs, for example, is somewhere around 32-34, and there was a guy in my A-100 training class that was 57 and just starting as an entry level FSO after teaching English, working for an airline, and then working for state government. The FSO hiring process is different than a lot of federal, private sector, and NGO hiring processes, but I've met people working in a lot of different jobs that have changed direction at some point in their lives.
I completed an MPP before becoming a FSO and I had classmates in their late 20s and early 30s who were hired by DOD and IC agencies. For example, one person from my degree program hired by DOD in her early 30s was a teacher before getting her MPP, so did not have a ton of prior IR/IC/military/etc experience. Again, there might be more of a tradeoff salary-wise and your film experience might not help you get a better paid or more senior position, but it's certainly not impossible.
There are tons of freshly minted MA IR (and similar degrees) running around and many of them have relatively interchangeable skills. However, not every MA IR grad will have a film background. That won't help for all jobs, but there are probably some employers who will see that as an advantage and it could open some extra doors.
I've known FSOs who have been teachers, publicists, firefighters, lawyers, insurance salespeople, investment bankers and journalists before becoming FSOs, and some who were hired right after undergrad. I've also worked with people from other USG agencies both in DC and overseas and while I don't conduct a full background check on everyone I work with, I've been impressed by the variety of backgrounds they have when the topic does come up.
One thing that I've observed both in the Foreign Service and in life in general is that there are many paths from point A to point B. Some might be more difficult, but there are many paths, nonetheless. So, best of luck with whichever path you choose.