Node run security checks before uploading to s3

Node run security checks before uploading to s3

I'm working on a way to upload a file from node to s3 using Knox running
on heroku or openshift. First, since I'm trying to get a direct stream
from the users computer to s3, are security scans(like mime type) , even
important? Second, since I will have about 100 mb of space on my heroku
instance, is it even possible to run scans without the file directly on my
server? Could I scan the file once it is on s3 without entirely
downloading it? Third, if a user get's a virus from a file uploaded to my
site, could I be considared liable? Thanks for your responses!