At the moment, this isn't something that can be done through Dropbox.
We currently have a request for a similar feature to what you mentioned here, which I can suggest sharing your support for by giving it a vote.
If you'd like to suggest something a little different, please feel free to submit your own suggestion there too. Another option I can suggest is to use a third party app which you might be able to find with a quick web search.
Sorry that I couldn't be of much more help with this, but please do let me know if you have any other questions.