Currently, everytime email content is saved to workspace its filename will follow this pattern:

triggername-buildidrandomnumber.[txt|html]

instead of specified in label description

Check this to enable saving the generated email content to a file in the workspace. The file will be named "triggername-buildid.[txt|html].

This should be changed as BUILD_ID actually represents BUILD_NUMBER and it's unique. Also some build process may need to know filename in order to be passed to other plugins.