Attachment helper for base64 encoding

[thinkingserious]

We have an example in USE_CASES.md on how to handle attachments.

We need to update the Attachment object such that it can also take in a raw file, rather than a base64 encoded file, and perform the base64 encoding behind the scenes.

Note that the total size of your email, including attachments, must be less than 30MB. The total size is calculated from the entire JSON payload. Note that you will need to handle the case where UTF-8 characters are included.

[thinkingserious]

Since there has been no activity on this issue since March 1, 2020, we are closing this issue. Please feel free to reopen or create a new issue if you still require assistance. Thank you!