You're using the Silverlight UI and when you try to add an attachment in the Shopping Cart, the Browse button is showing as inactive and doesn't let you use it.
SAP Business ByDesign Silverlight UI on Mac High Sierra OS 10.13+ with Safari 11
The issue is caused by an incompatibility of Microsoft Silverlight Plugin which is no longer supported for Mac devices.
This is a known limitation.
Mac, High Sierra, Browse, Shopping Cart, Safari , KBA , AP-RC-UIF-RT , UI Framework Runtime , Problem