Enable Groovy syntax highlighting for Jenkinsfile

### Description

It would be nice to have Groovy syntax highlightning for [Jenkinsfile](https://www.jenkins.io/doc/book/pipeline/jenkinsfile/), but they do not have an extension, so it is not possible to set the Groovy mimetype for them under Option -> Miscellaneous -> Files -> File Associations

### Use case/motivation

Example without higlight:
Example with:

### Related issues

_No response_

### Are you willing to submit a pull request?

No

Start with the NetBeans File Associations path named in the issue: Option → Miscellaneous → Files → File Associations, and inspect how a Jenkinsfile is identified when it has no extension. Trace the existing Groovy MIME-type and syntax-highlighting registration, then verify that Jenkinsfile content receives Groovy highlighting and that ordinary file associations remain unchanged.

Written by the indexing model from the issue text.