Slack: "no_file_data" error when forgetting to supply the Filename field when using a Reader

main.go

package main

import (
	"fmt"
	"os"

	"github.com/nlopes/slack"
)

func main() {
	api := slack.New(os.Getenv("SLACK_TOKEN"))
	fileParams := slack.FileUploadParameters{
		File:     "my-image.png",
		Channels: []string{"somechannel"},
	}
	if _, err := api.UploadFile(fileParams); err != nil {
		fmt.Printf("[fileParams] slack error: %v", err)
		// works as expected
		return
	}

	myImage, err := os.Open("my-image.png")
	if err != nil {
		fmt.Printf("error opening file: %v", err)
		os.Exit(1)
	}
	readerParams := slack.FileUploadParameters{
		Reader:   myImage,
		Channels: []string{"somechannel"},
	}
	if _, err := api.UploadFile(readerParams); err != nil {
		fmt.Printf("[readerParams] slack error: %v", err)
		// fails with "no_file_data"
		return
	}
}