Even if you got this to work, Dropbox is not a source control system, it only keeps 30 days of history for each file by default, doesn’t store descriptions of what actually changed, has no concept of changesets, and has no UnrealEd integration so you can’t visually diff blueprints. Best to spend some time (and if necessary money) now setting up a proper system, than kicking yourself later, especially if you have multiple people working on your project.
There is built-in support for Perforce and SVN, the former has a free usage tier that may be sufficient for you, the later is free. Epic primarily uses Perforce so that’s going to be the most robust and well tested option. Mercurial is another possible option, integration with UnrealEd can be achieved via my plugin, but unlike Perfoce and SVN there is no file locking which means you have to manage the work to ensure multiple people aren’t modifying the same asset at the same time. There is also a Git plugin in the works, but development only started recently, and it probably won’t be doing any locking either.
Whatever system you go with I’d suggest storing raw in a separate repository from the rest of the UE project files.