Zenkit

You can connect Zenkit directly to your Google Drive, Dropbox, and Box accounts.

Each time you upload a file into Zenkit by clicking "Add a file...", you can choose whether to add a file directly (using drag & drop or by browsing your computer), or you can select files from connected accounts.
To connect an account, choose the service you want to connect by clicking the button. A small window will open and prompt you to log in or allow access. Once you're logged in, you can select a file from your chosen app. Depending on the app, you can search for different documents, apply filters, sort items and change views. When you've chosen, just click "Select" or "Choose" in order to attach the file to your item.