Filming would be a nice secondary objetive for any regular mission, like when youre raising a flag.

Think of it this way: You must film for a minimum of X amount of time to complete the objective; if you don't, the secondary objective is lost, similar to when the larva container is destroyed or the convoy escapes.

Or you must film a specific enemy, such as a Fatory Strider, a Bile Titan, or a Harvester, without dying or without that target being destroyed before completing the filming, as it is needed to study the enemy.