Film permits are necessary in Buenos Aires if you plan on filming with a professional crew that will cause disruption to auto or pedestrian traffic, or if special effects will be used or action happen that can confuse or endanger passersby (for example, an actor uses a gun), or you need to park trucks, use a generator, etc. They are not necessary for a small documentary crew to film in the street, if that crew would cause no more disruption than a typical news crew.
While the process is your normal red-tape, be warned, you need at least 5 business days (and beware of holidays!) to get the first part, which is habitation to work, showing that you have insurance for crew and passersby–this is the hard part, and it is recommended to contract a local fixer or local production company to handle this. Then it is at least another 5 days to get permission for the specific location. And of course, all in Spanish!
The form that you need to fill out: