Newly implemented Email Template Testing feature doesn't work for Jelly scripts.
If name of template file is passed with extension following error occurs:
If name of template file is passed without extension following error occurs:
additionally a validation error is shown:
The problem is that EmailExtTemplateAction.doTemplateFileCheck() and EmailExtTemplateAction.renderTemplate() methods expect that template name includes extension. But JellyScriptContent.getTemplateInputStream() expects that template name doesn't contains extension.