Firstly, I have a feeleing, that your language preference should not influence the choice of db much. However, you haven't specified other criteria, so I will follow the points about the java-prefernece and the multimedia-idea.
You did not mention the purpose od your app, but you mention you would store multimedia. I have therefore a feeling that your question can be split in two fields: the architecture for storing multimedia is one part, db backend is the other part.
- The db choice according to the info you provided would be best related to your experience and your java preference.
- The multimedia store could be separate from the db (images and videos, and all other binary data) in files named by some tag, and metadata including descriptions in the database.
Separating multimedia from the database makes sense; why using the db as a filesystem? When there is much data to store (that is not bound to much other objects or structure), fielsystem is better suited. The db puts "more wight" on data relations, so keeping just metadata would be more efficient, whereas the storage-function can be outsourced to the filesystem.
Really not enough info to deliberate on the type od database. I also am not sure of how to fit restFul Api in it at this point. What I suggested is definitely ment for a learner, experimenting geek, but probably is not a full answer for building up a youtube replacement.