Your app is simple to make and the features is easy to implement.
The manager features like remove, rename or share can be done in 1 day.
The increased complexity is about open the files in others apps, check if the user has the needed app to open the file and optimize the performance searching the files.
Fortunately, I have experience with SQLite, the tool that solve this pŕoblemens and all this tricky part.
About me:
I work in Android Development since 2013, I have made apps to Volkswagen, Ford and I have won the World Skills America 2014 Bogota in Mobile Application Development