If you want IDEA to recognize a Jenkinsfile as a Groovy file, then you can add the String "Jenkinsfile" as a valid file name pattern (normally contains file endings) for Groovy files. This is supported "out of the box" without requiring any additional Plugin (except the "Groovy" Plugin, but that is already part of IDEA).
To do that go to the settings menu, open the "Editor" item and then "File Types". Now select "Groovy" in the upper list and add "Jenkinsfile". You can also use a regex like "Jenkinsfile*" if you want to be more flexible regarding an optional file ending for the Jenkinsfile.
The setting should now look like this:
So IDEA now provides syntax highlighting and auto completion as far as I can tell. It suggests existing function/method names while writing, but I'm not a Groovy developer, thus I can't tell if some suggestions are missing.