Bug: FormData is empty and request fails with HTTP 412 in v3 when using SetFiles()

[liyuan1125]

🐛 Bug: FormData is empty and request fails with HTTP 412 in v3 when using SetFiles()

🔁 Versions Tested
• Resty v2 (latest stable): ✅ Works
• Resty v3.0.0-beta.4: ❌ Fails with 412 Precondition Failed

package main

import (
	"fmt"
	"github.com/go-resty/resty/v2" // or v3
)

func main() {
	client := resty.New()

	addr := "https://api.weixin.qq.com/cgi-bin/media/uploadimg?access_token=xxx"

	resp, err := client.R().
		SetFiles(map[string]string{
			"media": "./media.jpg",
		}).
		Post(addr)

	if err != nil {
		fmt.Println("error:", err)
		return
	}

	fmt.Println("formData:", resp.Request.FormData)
	fmt.Println("statusCode", resp.StatusCode())
}

✅ Output (v2):
formData: map[@media:[./media.jpg]]
statusCode: 200

❌ Output (v3.0.0-beta.3)
formData: map[]
statusCode: 412

[liyuan1125]

Please verify multipart handling and ensure SetFiles() behaves consistently between versions.

Let me know if a reproducible curl or raw HTTP comparison is needed — I can provide it.

Thanks!

[liyuan1125]

Due to this issue, our production system experienced unexpected failures when using resty v3. The affected endpoint (WeChat image upload API) is critical to our business.

As a result, we were forced to roll back to resty v2 to ensure system stability.

Please prioritize this regression — we’d like to adopt v3, but this blocker prevents us from upgrading.