It is useful to send HTML e-mails with a plaintext alternative. Using HTML-only mails causes SpamAssassin to increase the spam score for those e-mails via the rule MIME_HTML_ONLY BODY.

The sending of HTML mails with automatically generated plaintext alternatives was introduced with JENKINS-23126 . It is possible to configure a project to use a Content Type of "Both HTML and Plain Text". However, this option isn't available on the Jenkins-wide configuration page (i.e. the one reachable via "Manage System").

 This seems like an oversight to me. Allowing this option on the system-wide page would allow all projects to inherit it by selecting the choice "Default Content Type" for Content Type in their configuration.