Newly implemented Email Template Testing feature doesn't work for Jelly scripts.

If name of template file is passed with extension following error occurs:

Jelly script [myscript.jelly] was not found in $JENKINS_HOME/email-templates.

If name of template file is passed without extension following error occurs:

Template [myscript] was not found in $JENKINS_HOME/email-templates.

additionally a validation error is shown:

The file 'myscript' does not exist

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.