Q: Attachments in mail - 2016 still a problem....

I Can not replicate this behavior. It works exactly as expected on my iPad.

I don't think that the problem is with the email account, but have you tried deleting the mail account and adding it back again?

Have you reset all settings? Settings>General>Reset>Reset all settings. This returns the device to out of the box condition with regard to all settings including WiFi passwords. All settings will have to be entered again.

Have you restored to factory settings? Backup, restore to factory settings, then restore the backup.

Use iTunes on your Mac or PC to restore your iPhone, iPad, or iPod to factory settings - Apple Support

I prefer using iTunes since that will put a clean copy of the iOS on the device after you erase it. If you prefer, erase all content and settings, then restore a backup.

Erase all content and settings on your iPhone, iPad, iPod touch, or Apple Watch - Apple Support

Always backup first.

How to back up your iPhone, iPad, and iPod touch - Apple Support

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support