Adds MimeType to Attachment struct

[CharlesWinter]

It's present in the raw JSON, just not on the actual type.

Submit a pull request

CLA

• [ X] I have signed the Stream CLA (required).
• [ X] The code changes follow best practices
• [ X] Code changes are tested (add some information if not applicable)

Description of the pull request

Adds the missing MimeType field to the Attachments struct. It's present in the returned JSON from the webhook.

[CharlesWinter]

@totalimmersion @JimmyPettersson85 Hello! 👋 is there something more I have to do to get this reviewed and hopefully approved? I couldn't see anything obvious in the PR guidelines.