What data "MUST" be stored inside a "relational database"?

I am working on a mobile/web app like Instagram. Considering that, I want to know for what part of the app it's better to use relational and for what other parts non-relational databases. I have chosen MySQL and Cassandra databases and this is my research result so far: -Relational Databases: - For services we need as much as possible consistency like payment service or ordering and authentication services. - Non-relational Databases: - For services we need to summarize a lot of different data in a small table instead of a lot of columns with null values, like user services that we have a lot of personal settings. - For the time we need horizontal scalability and want more distributed system over different datacenters/clouds. - For faster read/write heavy systems. But I am still wondering to know: 1- What database is more appropriate for upload files(images, videos. documents) on it? 2- What database is more sufficient for posts/comments/likes, list of friends, and other user related things that they are also related to other users. (***I mean a post or a comment is related to one user but all the other users can also see it, and may affect them***).