In your Conversation Flow, go the the specific step you need to add the files into. Click "Upload Files" at the right panel.
Option 1:
After clicking "Upload Files", you will see this upload box.
|  |
"Drag and drop" the files from your Windows Explorer into the file upload box. |  |
Click "Submit". |  |
Option 2:
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". | 
|
Click "Submit". | 
|