Mailbox listener computing the preview for a given mail (attachments + mime parts)

[quantranhong1999]

Why

We need to asynchronously compute the thumbnail for attachments + mime parts upon newly added mails.

How

Upon the newly added event, get the attachments, call the thumbnail server (using the thumbnail client), receive the result, and store it using the thumbnail storage APIs.

DoD

• Listener + unit test
• Make sure thumbnail is computed and stored upon new mail
• Make sure thumbnail is computed for both attachments and mime parts (inline attachments?) case