Incorrect MIME Type for .mjs files served (Expected application/javascript, got application/octet-stream)

## Affected Puppet, Ruby, OS and module versions/distributions

- Puppet: *
- Ruby: *
- Distribution: *
- Module version: *

## How to reproduce (e.g Puppet code you use)
Setup nginx to serve mjs files

## What are you seeing
The mjs files have a mime type of `application/octet-stream`

## What behaviour did you expect instead
The mime type should be `application/javascript`
