UploadFileV2 uploads incorrectly when using text Content parameter

[calebmckay]

What happened

When calling UploadFileV2() with a string passed to the Content parameter, the file is incorrectly uploaded. This seems to be because old files.upload endpoint allowed text content to be passed as a POST parameter, but the new upload URL interprets the unexpected content= parameter as the start of the file content, and doesn't correctly URL-decode it.

Expected behavior

Steps to reproduce

reproducible code

package main

import (
	"os"

	"github.com/slack-go/slack"
)

func main() {
	token := os.Getenv("SLACK_AUTH_TOKEN")
	channelId := os.Getenv("SLACK_CHANNEL_ID")
	client := slack.New(token, slack.OptionDebug(true))

	fileContent := "This is test file content"
	fileSize := len(fileContent)

	fileParams := slack.UploadFileV2Parameters{
		Channel:        channelId,
		Content:        fileContent,
		Filename:       "file.txt",
		FileSize:       fileSize,
		InitialComment: "Test file",
		Title:          "Test file",
	}
	client.UploadFileV2(fileParams)
}

This seems to only be an issue with the Content parameter, as converting the string to a Reader fixes the problem:

reader := strings.NewReader(fileContent)

fileParams := slack.UploadFileV2Parameters{
	Channel:        channelId,
	Reader:         reader,
	Filename:       "file.txt",
	FileSize:       fileSize,
	InitialComment: "Test file",
	Title:          "Test file",
}
client.UploadFileV2(fileParams)

Versions

• Go: go version go1.20.7 darwin/arm64
• slack-go/slack: v0.13.0