Understanding the File Upload API

Oct 13, 2025 by wbrum.org 2 views

Introduction

In this article, we will explore the issue of uploading a PDF file to Slack using the latest file upload API in Golang. The file is uploaded successfully, but it appears as a binary file in the Slack preview. We will investigate the possible causes of this issue and provide a solution to fix it.

Understanding the File Upload API

The file upload API in Slack is a two-step process. The first step is to get an upload URL using the getUploadURLExternal API call. This API call returns a URL that can be used to upload the file. The second step is to upload the file to the URL returned in the first step.

Step 1: Get Upload URL External

The getUploadURLExternal API call is used to get the upload URL. This API call returns a JSON response that contains the upload URL and other metadata.

import (
	"encoding/json"
	"fmt"
	"io/ioutil"
	"net/http"
)

type UploadURLResponse struct {
	Url string `json:"url"`
}

func getUploadURLExternal(token string) (string, error) {
	req, err := http.NewRequest("GET", "https://slack.com/api/files.uploadExternal", nil)
	if err != nil {
		return "", err
	}
	req.Header.Set("Authorization", "Bearer "+token)
	req.Header.Set("Content-Type", "application/json")

	resp, err := http.DefaultClient.Do(req)
	if err != nil {
		return "", err
	}
	defer resp.Body.Close()

	body, err := ioutil.ReadAll(resp.Body)
	if err != nil {
		return "", err
	}

	var response UploadURLResponse
	err = json.Unmarshal(body, &response)
	if err != nil {
		return "", err
	}

	return response.Url, nil
}

Step 2: Upload File

The second API call is used to upload the file to the URL returned in the first step. This API call returns a JSON response that contains the uploaded file metadata.

import (
	"encoding/json"
	"fmt"
	"io/ioutil"
	"net/http"
)

type UploadFileResponse struct {
	File struct {
		ID       string `json:"id"`
		Name     string `json:"name"`
		Type     string `json:"type"`
		Filetype string `json:"filetype"`
		Mimetype string `json:"mimetype"`
	} `json:"file"`
}

func uploadFile(token string, url string, file *os.File) (string, error) {
	req, err := http.NewRequest("POST", url, file)
	if err != nil {
		return "", err
	}
	req.Header.Set("Authorization", "Bearer "+token)
	req.Header.Set("Content-Type", "application/octet-stream")

	resp, err := http.DefaultClient.Do(req)
	if err != nil {
		return "", err
	}
	defer resp.Body.Close()

	body, err := ioutil.ReadAll(resp.Body)
	if err != nil {
		return "", err
	}

	var response UploadFileResponse
	err = json.Unmarshal(body, &response)
	if err != nil {
		return "", err
	}

	return response.File.ID, nil
}

Issue with PDF File Upload

The issue with uploading a PDF file to Slack is that it appears as a binary file in the Slack preview. However, once downloaded, it is in the correct PDF format. The filetype and mimetype values extracted from the completeExternalUrl response seem inconsistent.

Possible Causes

There are several possible causes of this issue:

The filetype and mimetype values are not being set correctly in the uploadFile API call.
The Content-Type header is not being set correctly in the uploadFile API call.
The file is not being uploaded correctly to the URL returned in the getUploadURLExternal API call.

Solution

To fix this issue, we need to ensure that the filetype and mimetype values are set correctly in the uploadFile API call. We also need to ensure that the Content-Type header is set correctly in the uploadFile API call.

Set Filetype and Mimetype

We can set the filetype and mimetype values in the uploadFile API call by passing them as parameters to the API call.

func uploadFile(token string, url string, file *os.File, filetype string, mimetype string) (string, error) {
	req, err := http.NewRequest("POST", url, file)
	if err != nil {
		return "", err
	}
	req.Header.Set("Authorization", "Bearer "+token)
	req.Header.Set("Content-Type", mimetype)

	resp, err := http.DefaultClient.Do(req)
	if err != nil {
		return "", err
	}
	defer resp.Body.Close()

	body, err := ioutil.ReadAll(resp.Body)
	if err != nil {
		return "", err
	}

	var response UploadFileResponse
	err = json.Unmarshal(body, &response)
	if err != nil {
		return "", err
	}

	return response.File.ID, nil
}

Set Content-Type Header

We can set the Content-Type header in the uploadFile API call by passing the mimetype value as a parameter to the API call.

func uploadFile(token string, url string, file *os.File, mimetype string) (string, error) {
	req, err := http.NewRequest("POST", url, file)
	if err != nil {
		return "", err
	}
	req.Header.Set("Authorization", "Bearer "+token)
	req.Header.Set("Content-Type", mimetype)

	resp, err := http.DefaultClient.Do(req)
	if err != nil {
		return "", err
	}
	defer resp.Body.Close()

	body, err := ioutil.ReadAll(resp.Body)
	if err != nil {
		return "", err
	}

	var response UploadFileResponse
	err = json.Unmarshal(body, &response)
	if err != nil {
		return "", err
	}

	return response.File.ID, nil
}

Conclusion

In this article, we explored the issue of uploading a PDF file to Slack using the latest file upload API in Golang. We investigated the possible causes of this issue and provided a solution to fix it. We set the filetype and mimetype values in the uploadFile API call and set the Content-Type header correctly. We also provided the documentation for the second API call used to upload the file after the getUploadURLExternal API call.

Documentation for the Second API Call

The second API call used to upload the file after the getUploadURLExternal API call is the uploadFile API call. This API call returns a JSON response that contains the uploaded file metadata.

API Call Parameters

The uploadFile API call takes the following parameters:

token: The Slack token used to authenticate the API call.
url: The URL returned in the getUploadURLExternal API call.
file: The file to be uploaded.
filetype: The type of the file.
mimetype: The mimetype of the file.

API Call Response

The uploadFile API call returns a JSON response that contains the uploaded file metadata. The response contains the following fields:

file: The uploaded file metadata.
id: The ID of the uploaded file.
name: The name of the uploaded file.
type: The type of the uploaded file.
filetype: The type of the uploaded file.
mimetype: The mimetype of the uploaded file.

Example Use Case

Here is an example use case of the uploadFile API call:

func main() {
	token := "xoxb-1234567890-abcdefghijklmnopqrstuvwxyz"
	url, err := getUploadURLExternal(token)
	if err != nil {
		fmt.Println(err)
		return
	}

	file, err := os.Open("example.pdf")
	if err != nil {
		fmt.Println(err)
		return
	}

	id, err := uploadFile(token, url, file, "pdf", "application/pdf")
	if err != nil {
		fmt.Println(err)
		return
	}

	fmt.Println(id)
}

This example use case demonstrates how to use the uploadFile API call to upload a PDF file to Slack. The getUploadURLExternal API call is used to get the upload URL, and the uploadFile API call is used to upload the file to the URL. The filetype and mimetype values are set correctly in the uploadFile API call, and the Content-Type header is set correctly. The uploaded file metadata is returned in the API call response.

Introduction

In our previous article, we explored the issue of uploading a PDF file to Slack using the latest file upload API in Golang. We investigated the possible causes of this issue and provided a solution to fix it. In this article, we will answer some frequently asked questions related to the file upload API in Slack.

Q: What is the file upload API in Slack?

A: The file upload API in Slack is a two-step process. The first step is to get an upload URL using the getUploadURLExternal API call. This API call returns a URL that can be used to upload the file. The second step is to upload the file to the URL returned in the first step.

Q: What is the `getUploadURLExternal` API call?

A: The getUploadURLExternal API call is used to get the upload URL. This API call returns a JSON response that contains the upload URL and other metadata.

Q: What is the `uploadFile` API call?

A: The uploadFile API call is used to upload the file to the URL returned in the getUploadURLExternal API call. This API call returns a JSON response that contains the uploaded file metadata.

Q: What are the parameters of the `uploadFile` API call?

A: The uploadFile API call takes the following parameters:

token: The Slack token used to authenticate the API call.
url: The URL returned in the getUploadURLExternal API call.
file: The file to be uploaded.
filetype: The type of the file.
mimetype: The mimetype of the file.

Q: What is the response of the `uploadFile` API call?

A: The uploadFile API call returns a JSON response that contains the uploaded file metadata. The response contains the following fields:

file: The uploaded file metadata.
id: The ID of the uploaded file.
name: The name of the uploaded file.
type: The type of the uploaded file.
filetype: The type of the uploaded file.
mimetype: The mimetype of the uploaded file.

Q: How do I set the `filetype` and `mimetype` values in the `uploadFile` API call?

A: You can set the filetype and mimetype values in the uploadFile API call by passing them as parameters to the API call.

Q: How do I set the `Content-Type` header in the `uploadFile` API call?

A: You can set the Content-Type header in the uploadFile API call by passing the mimetype value as a parameter to the API call.

Q: What is the example use case of the `uploadFile` API call?

A: Here is an example use case of the uploadFile API call:

func main() {
	token := "xoxb-1234567890-abcdefghijklmnopqrstuvwxyz"
	url, err := getUploadURLExternal(token)
	if err != nil {
		fmt.Println(err)
		return
	}

	file, err := os.Open("example.pdf")
	if err != nil {
		fmt.Println(err)
		return
	}

	id, err := uploadFile(token, url, file, "pdf", "application/pdf")
	if err != nil {
		fmt.Println(err)
		return
	}

	fmt.Println(id)
}

Q: What are the possible causes of the issue of uploading a PDF file to Slack?

A: There are several possible causes of this issue:

The filetype and mimetype values are not being set correctly in the uploadFile API call.
The Content-Type header is not being set correctly in the uploadFile API call.
The file is not being uploaded correctly to the URL returned in the getUploadURLExternal API call.

Q: How do I fix the issue of uploading a PDF file to Slack?

A: To fix this issue, you need to ensure that the filetype and mimetype values are set correctly in the uploadFile API call. You also need to ensure that the Content-Type header is set correctly in the uploadFile API call.

Q: What is the documentation for the second API call used to upload the file after the `getUploadURLExternal` API call?

A: The second API call used to upload the file after the getUploadURLExternal API call is the uploadFile API call. This API call returns a JSON response that contains the uploaded file metadata.