Users are unable to download email artifacts as .zip. When a user selects an email artifact and clicks "Download > Download as ZIP" The email artifact is downloaded as a .eml (Microsoft Email Message). They confirmed that they are also unable to download email artifacts as .zip. This also looks to be the case with other artifact types as well i.e. text/html, application.
This is a function of the browser unzipping the file as it is downloaded and processed.
The browser or the desktop O/S was unzipping the file as it was saved to disk. Safari unzips files by default as they are downloaded. This can be disabled in the System Preferences for Safari. Select the General icon and scroll down to "Open safe files after downloading" and uncheck the box.
This may also be true for Chrome and Firefox.