Introduction pages are broken

[matt0x6F]

Describe the bug

The bug is two fold:

1. Introduction pages do not appear to load the file from disk the way the configuration claims. There's tests that assert this behavior and I've simulated it locally, but when I ran it in production it doesn't seem to work.
2. The Content-Type header that Chrome receives first is the one that is honored. In our case, the right content type is sent, but it's second because of an existing bug in Athens. Firefox doesn't seem to do this, it takes the last Content-Type header.

Error Message
The content is rendered as application/json and the in-memory template is loaded even when a file is specified.

To Reproduce

1. Specify a location with https://github.com/gomods/athens/blob/main/config.dev.toml#L174

2. Launch an image

3. curl -v the root endpoint

Expected behavior

An HTML page is shown, if specified the custom one, and it is rendered as HTMl by the browser.

Environment (please complete the following information):

• OS: Linux 64bit
• Go version : 1.22
• Proxy version : v0.14.0
• Storage (fs/mongodb/s3 etc.) : FS

Additional context
N/A