Update custom header example to more relevant example

[gilzow]

Where on docs.platform.sh should be changed?

https://docs.platform.sh/create-apps/web/custom-headers.html
https://docs.upsun.com/create-apps/web/custom-headers.html

What exactly should be updated?

Current example uses .mp3 as the example file type but mp3 is already a known mime type in the nginx.conf

[email protected]:~$ cat /etc/nginx/mime.types | grep mp3
    audio/mpeg                                       mp3;

Instead, let's use a file type that isn't known that might need to use the customer header to set the correct content-type (e.g. *.md)

Additional context

I think we should also add links to this page from https://docs.upsun.com/create-apps/app-reference/single-runtime-image.html#locations and https://docs.upsun.com/create-apps/app-reference/composable-image.html#locations (and platform.sh equivalents) in the Description field for headers