Mobile:
Go to Assets tab
Select Locations
Select Location to add File
Click Edit in top right corner
Click Add in Files field
Select Camera, My Photos, or My Files
Camera: Take picture using camera on device
My Photos: Choose from existing photos on device
My Files: Choose existing file on device
Click Update in top right corner
Web:
Go to Locations tab
Select Location to add File
Click Edit in top right corner
Click Attach files
Choose file to add
Click Update in bottom right corner