Initial implementation based on email-to-pdf-converter

[ChristophWurst]

Feature request

As a Nextcloud user I want to view the contents of an .eml or .msg file. These might be sent via an email, so I open them with Viewer in Mail, or they are shared via Nextcloud Files and I open them with Viewer as well.

Implementation

An .eml (message/rfc822) or .msg (application/vnd.ms-outlook) file can't just be opened by the browser, but it can be converted to a viewable representation. The project https://github.com/nickrussler/email-to-pdf-converter offers a Java tool that converts the two file types into a PDF in a few seconds. This Nextcloud app can add a simple route to which a .eml/.msg file can be sent and converted. Sending the file makes the feature usable for attachments in Mail and also files on DAV.

Work packages

• Create an endpoint for the conversation
• Extend viewer with a new handler but reuse the existing PDF handler
• Documentation
  • Requirements: files_pdfviewer installed, JRE, wkhtmltopdf
  • Security: usage of wkhtmltopdf

[ChristophWurst]

Re nextcloud/mail#9078. This feature should be standalone inside this app

The app shall work in a way that eml files can be opened anywhere in Nextcloud, be it Files, Talk or Mail. The feature is not specific to opening Mail attachments.