Bugbug email attachment assertion

It would be nice to be able to assert that an email has an attachment and is equal to a file, with an option to upload the file to check against like you would for an input.

We send out some attachments gathered from a couple different external sources, and want to ensure that no changes on their end cause us to send corrupted or blank attachments. We are currently using the "View Raw" option on the email client and verifying the first few lines of B64 encoded bytes, but the tests freeze up if the attachments are large (>1MB).