Download URLs always save file as `openapi.json`

Describe the bug
A custom download URL will always attempt to save the file as openapi.json. This is problematic if the download URL is e.g. a YAML file.

Expected behavior
A custom download URL does not force the file to be downloaded with the name openapi.json.

Minimal reproducible OpenAPI snippet(if possible)

Use any OpenAPI YAML.

redocly.yaml:

apis:
  example:
    root: ./example-original.yaml
    openapi:
      downloadUrls:
        - title: Download OpenAPI spec YAML
          url: './example-original.yaml'

Generate the HTML. The download button will save a file called openapi.json, where I would expect example-original.yaml

Screenshots

Additional context

Version: 8.0.4033.34 (Official Build) (arm64)
Chromium Version: 148.0.7778.183
Channel: Official Build
Platform / OS: macOS Version 26.4 (Build 25E246)

Generate the HTML from the provided redocly.yaml using a custom OpenAPI YAML download URL, then inspect the download button behavior. Trace how the custom URL is handled and ensure the downloaded file uses the URL's filename rather than openapi.json, including for example-original.yaml.