Added ability to download .eml file for the email preview.

[igorkasyanchuk]

Motivation / Background

It could be useful for developers to download EML file for email. Currently, I'm using mailcatcher or maildev to send emails and download ".eml" file there. So I can open it in Outlook for example and see how it looks.

PS:
This is a copy of this PR #44969. That branch is corrupted now, I'm too tired to solve the git issue. Trying a new branch.

Detail

This Pull Request changes [REPLACE ME]

Additional information

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• There are no typos in commit messages and comments.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Feature branch is up-to-date with main (if not - rebase it).
• Pull request only contains one commit for bug fixes and small features. If it's a larger feature, multiple commits are permitted but must be descriptive.
• Tests are added if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.
• PR is not in a draft state.
• CI is passing.