Right click an attachment and you'll get a menu. This menu used to have more mnemonics. Currently there's only a mnemonic for "Open With" but in KMail 4.14.9 there used mnemonics for all menu entries! (Open, Open With, View, Save As, Copy, Delete Attachment, Properties) Reproducible: Always
This issue seems either intentional or an old (code) bug to me. Most likely intentional; KMail was probably at a transition period between KDE4 and KDE Plasma 5, since now I do see several context menu options: Open, Open with, Scroll to, Save as, Copy, Delete attachment, Properties.