How to get a file from drop box app in order to upload it on external server?

Hi all ionic folks,
I like to do the following:
1 i do not want to use dropbox api
2 i have a button on my app saying "upload a file from dropbox"
3 after tap on it, i like drop box app to open to choose a file
4 after choosing in drop box, come back to my app directly and use this file for uploading to any other external server ( i can keep this file for a while) on my app

Trello mobile app use this scenario in order to attach a file to a card - if you are familiar.
Id like to have exactly the same behaviour

Have you got any idea how i can achieve it.
Think you