What Linux programs does Process::openEmailWithAttachments()?
None - it’s not supported as of yet.
I’ve tried this on Windows with Thunderbird. JUCE creates a template for sending, but the send button must be pressed manually. This is inconvenient without full automation of sending.
There would be a severe security issue if mail clients allowed such transparent “automation”. Malicious code could use it to send spam. If you want to actually send an email (with attachment or not) you might have to do the hard work yourself (SMTP protocol handling, etc), or find a suitable library for that.