As you know, Dropbox does not offer self hosted server option. So if you want to have your own private 'Dropbox' - consider Bitrix24.
It's free for 12 users and, unlike Dropbox, it’s available both in cloud and as self hosted software (you get open source access if you select this option).
Importantly, you'll be able to use Bitrix24 mobile and desktop apps when you go for on premise deployment. Assess to files can be set up according to your
company structure and Bitrix24 comes with a number of document approval workflows that no one else has. And since self hosted Bitrix24 comes with CMS and
open source code access, you can insert your own logo, change appearance and do much more.
If you want to start using free online file sharing server, this Bitrix24 manual will get you up to speed quickly.
In order to test self hosted (on premise) edition of Bitrix24, you need to download it from here. Please keep in mind that it takes about 1.5 hours to deploy and set it up.
Self hosted Bitrix24 manual is available here.