You can upload assets from two types of sources: local storage, or cloud storage.
Local storage
You can upload assets from any source accessible to your computer, including internal drives, network drives, external hard drives, thumb drives, optical drives, and SD/XD cards. If you can access a file from Windows Explorer or the macOS Finder, you can upload that file as an asset to Extensis Connect.
Cloud storage
Connect uses Kloudless to allow you to easily utilize one or more cloud storage services, such as Google Drive, DropBox, and Box. When you choose a service, you will be asked to allow Kloudless to access that service. (You may also be prompted for your service login credentials.)
Once you access a service through Kloudless, your storage on that service will be immediately available to upload assets to Connect.
Accessing the first cloud service
To connect a cloud service for uploading assets, click 
Upload, then choose Cloud Storage.
In the Choose service dialog, click once on the service that you want to use.
The service you selected may prompt you to allow Kloudless to access your account. Depending on the service, you may need to enter your login credentials.
Once your cloud storage service is connected, you can proceed to upload assets or close the dialog.
Using additional cloud services
To use another cloud service, click Upload, choose Cloud Storage, click 
next to the selected cloud service, then choose 
Manage Accounts.
Click the service that you want to use. Allow Kloudless to access the service, and enter your login credentials if prompted.
Unsupported cloud services
If you need to upload assets from a cloud service that is not supported by Kloudless (such as iCloud Drive), temporarily copy the assets locally, upload from your local drive, then delete the local copies.
Be aware that some services, such as iCloud Drive, will move files (not copy them) when you drag from one drive to another.