Attach PDFs using "Content-disposition: inline" to avoid problems with macOS Mail
Some of my customers have reported that invoice emails do not have any attachments. It turns out they are all using macOS Mail, which no longer displays attached PDFs in a way that is very obvious for all users - there's a paperclip icon that indicates an attachment is present, and another paperclip icon which lets you open attachments, but this only appears when you hover over it.
If you send a PDF from within Mail, it attaches it using Content-Disposition: inline, which means that the invoice is displayed at the bottom of the message.
Could you please consider changing the attachment method, or adding an option to allow this to be tweaked? At the moment I have to manually re-send invoices to affected customers.