Mobile:
Go to Assets tab
Select Asset to add Asset Type
Click Edit in top right corner
Scroll to Asset Type
Click Add
Select existing Asset Type(s) or click Create Type to create new Asset Type(s)
Click Create
Click Done
Web:
Go to Assets tab
Select Asset to add Asset Type
Click Edit in top right corner
Type Asset Type(s) in Asset Type field
Click +Create
Click Update in bottom right corner