Dropbox won't work. Here is why:
Dropbox provides, through a very controlled user interface, file storage. Sumac does not store its data on a file server. Instead, it stores the data in a database management system (DBMS).
Some examples of DBMSs: MySQL, MS SQL, Oracle, DB2. We use MySQL. A DBMS is a program living on a server and managing data.
A program like Sumac sends commands to the DBMS (e.g. commands like "get all the donations this month" or "insert this new contact into the database"). The DBMS software interprets these commands, responds to them, and manages the database, storing it into a bunch of files.
So while ultimately there are underlying files holding the data, Sumac does not actually use them. Sumac talks to the DBMS which, in turn, uses the files.