How do I detect file type of input that's piped into a Go program that expects either images or gifs?

I have a cli tool that accepts input in the form of file paths or urls (which can be gifs or images). Currently, I'm differentiating btw the two by their extensions (.gif, .jpg etc at the end of the paths), but I'm stumped on how to do that if the input is piped from a different process in the command line.

For instance, how will myGoProgram know the piped input is a .png or a .gif in the example below?

cat image.png | ./myGoProgram -

Here's how I'm decoding piped stdin for an image using the image go module right now:

decodedImg, _, err := image.Decode(os.Stdin)

...And the same with a decoding a gif through the image/gif go module:

decodedGif, err = gif.DecodeAll(os.Stdin)

But I don't understand how I'm supposed to know if it's a gif or an image from os.Stdin

Was able to work out a solution with the help of the question referred by @mkopriva

Overall logic for how I differentiated btw images, gifs and invalid inputs:

// imports:
import (
	"io/ioutil"
	"log"
	"net/http"
	"os"
)

// -----
// logic:
var pipedInputTypes = []string{
	"image/png",
	"image/jpeg",
}
var inputIsGif bool

pipedInputBytes, err := ioutil.ReadAll(os.Stdin)
if err != nil {
	log.Fatalf("Unable to read piped input: %v", err)
}

fileType := http.DetectContentType(pipedInputBytes)
invalidInput := true

if fileType == "image/gif" {
	inputIsGif = true
	invalidInput = false
} else {
	for _, inputType := range pipedInputTypes {
		if fileType == inputType {
			invalidInput = false
			break
		}
	}
}

if invalidInput {
	log.Fatalf("Cannot read piped input of type \"%v\"", fileType)
}

if inputIsGif {
    // Gif program logic with pipedInputBytes
} else {
    // Image program logic with pipedInputBytes
}