Fix mime-type for json files to "application/json" to avoid errors in requesting applications

What does this MR do?

Json files are delivered by GitLab as content-type "text/plain" which is considered by some applications as error and they may refuse further processing of the content.

The standardized mime type for json files is "application/json".

This patch fix the mime type of the json files and allow to access those by exernal applications like Rundeck (Allowed Values > Remote URL with currently) with currently exits with the error:

Failed loading remote option values 
Exception: java.lang.Exception: Unexpected content type received: text/plain; charset=utf-8

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer