fix: missing plaintext with inline attachments

[CanRau]

This should fix #67

At least in my personal quick tests it works, let me know if you need me to do anything differently or if anything is missing I might not've considered 🤓

Your lib is really appreciated and made me learn a lot about mime ❤️

[naktinis]

@muratgozel do you think we could merge this PR?

[muratgozel]

:tada: This PR is included in version 3.0.27 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: