FEATURE: Allow searching for emails with (specific) attachments

[tillprochaska]

Is your feature request related to a problem? Please describe.

• I want to be able to filter for emails with attachments.
• I want to be able to filter for emails with specific attachments (e.g. based on file name or file extension).

Describe the solution you'd like

• schema:Email AND properties.attachments:* should return all emails that have any attachment.
• schema:Email AND properties.attachments:*.pdf should return all emails that have an attachment ending with .pdf.

Describe alternatives you've considered

• Searching for properties.mimeType:multipart/mixed might help narrowing down the list of emails, but every email that has HTML and plaintext alternatives will also be included by this even if it doesn’t have any attachments.

Additional context

• Email attachments are separate entities that have a reference to the email, but there’s no efficient way to run joins in ES.
• An easy solution probably would be to add the file names of attachments to the Email entity itself, e.g. in a new attachments property. This wouldn’t allow queries for arbitrary attachment properties, but likely cover many use cases.