[JS] Image URLs via {{media url=image}} fail after update — is this intended?

[7hokerz]

Describe the bug
After updating to the latest version(Genkit JS 1.27.0),
image inputs defined via dotPrompt using {{media url=image}} cause a runtime error when converted to OpenAI chat completion content.
This feature works fine in version 1.26.0 or lower.

The probable causes are the following changes: https://github.com/firebase/genkit/pull/3923/files

Full error message:

Error: File URLs are not supported for chat completions.
Only base64-encoded files and image URLs are supported.
Content type: undefined

To Reproduce

1. Define a prompt using dotPrompt with image media:

{{media url=image}}

2. Bind image to a remote image Signed-URL from Firebase Storage:

{
  image: {
    url: 'https://storage.googleapis.com/...'
  }
}

Runtime (please complete the following information):

• OS: windows 10 22H2

Node version

• v24.13.0

Additional context
It looks like the regression is caused by relying solely on part.media.contentType
to determine whether a media input is an image.
One possible way to resolve this could be to prioritize the declared media intent
(e.g. image) over contentType, or to add a fallback for common image URL patterns
when contentType is missing.