In your Conversation Flow, go the the specific step you need to add the files into. Click "Upload Files" at the right panel.
After clicking "Upload Files", you will see this upload box.
"Drag and drop" the files from your Windows Explorer into the file upload box.
From the same "File Upload" box, click "Browse Files". Choose your file source (ie: Desktop, Conversation, Google Drive, etc).
Select the file/s that needs to be added then click "Open".