Create MIME/Multipart message with attachments in WhisperBack

Feature Branch: https://gitlab.com/saschamarkus/whisperback/tree/feature/8722-Create-MIME-Multipart-attachments

Attachments

• test_8722_attached.eml
• test_8722_inline.eml
• 
• 0001-Attach-debug-info-as-individual-files.patch

Parent Task: #6183

Related issues

• Blocked by #8514 (closed)

Original created by @alant on 8722 (Redmine)