Since JENKINS-18203, it's possible to refer to files (template, script, etc...) to be used by email-ext-plugin and emailext-template-plugin from files defined through config-file-provider-plugin by using the "managed:" prefix.

The issue is that the "managed:" syntax is working fine only when such configuration is done at the top level of Jenkins.
Whereas the config-file-provider-plugin enables also to configure such files at folder level (cf. cloudbees-folder-plugin), with lowest folder level taking precedence in case of files with same name defined at several levels of the folder tree, this "managed:" syntax does not work in such case like it should.

I'm providing a solution in a GitHub pull-request with unit-tests in a separated commit, so that anyone can figure out the failing scenarii before the fix commit.