I've attached my own version which I had been working on before.
I think what happens here, is that Fifield and Milburn are alone in one of the caverns. Milburn falls in the black ooze, something comes up to him, he reaches to it and the worm wraps itself around his arm. He initially thinks it's okay, and he calms himself down. Then the thing gets tighter and tighter so he asks Fifield to help him take it off. Fifield is hesitant, but eventually tries. The worm breaks Milburns arm as Fifield tries to get it off. Milburn then frantically commands to "cut it off!". Then the thing gets into Milburns suit and reaches his face, whereby the thing melts that part of the mask as seen in the trailer. Then the thing launches itself out to infect Fifield.
This is my best guess anyways. You definitely hear acid melting stuff in the clip, and you hear something launch itself at Fifield and his reaction.