I was able to reproduce the problem. From searching this problem it seems iOS only shows attachments for “mixed” messages but not pure HTML messages:
Unfortunately, n8n’s “Send Email” node does not currently support such mixed messages. This appears to be problematic behaviour in iOS’s mail application, but it might still be worth for us to take a closer look into whether we can fix this. I’ll add it to our bug tracker so the engineering team can take a closer look into whether we can offer this option going forward.