Support uploading multiple files in a single request

[Syfaro]

Certain methods (sendMediaGroup, editMessageMedia, anything that supports thumbnails) allow uploading multiple files. Currently, UploadFile is structured in such a way it can only accept a single file to upload:

telegram-bot-api/bot.go

Lines 151 to 159 in da98517

	// UploadFile makes a request to the API with a file.
	//
	// Requires the parameter to hold the file not be in the params.
	// File should be a string to a file path, a FileBytes struct,
	// a FileReader struct, or a url.URL.
	//
	// Note that if your FileReader has a size set to -1, it will read
	// the file into memory to calculate a size.
	func (bot *BotAPI) UploadFile(endpoint string, params Params, fieldname string, file interface{}) (APIResponse, error) {

Therefore, there needs to be a change to support uploading multiple files. One blocking issue is that the underlying library used to send files without reading everything into memory, multipartstreamer, only accepts one file per request. A replacement would have to be created.

I would love to get feedback on the best way to implement this.

[HirbodBehnam]

Hello!
Before reading my comment keep in mind that I'm really noob.
I suggest using io.pipe with mime/multipart and io.copy to upload your files with minimal ram usage. (The buffer size of io.Copy is 32KB)
Here is a small code sample I write for multi-file uploads with comments on most of the lines to explain what is going on (as you can see you can even use bytes.NewReader to send data from memory):
https://gist.github.com/HirbodBehnam/d0344f2377cbcd96bc058f31360a7658
I hope it helps

Edit: If you think this is ok, tell me and I try to update the api with this approach and open a pull request.

[Syfaro]

Hi @HirbodBehnam, thank you for the code sample! That seems like a great strategy for doing multiple uploads.

However, because it involved a rather large internal change to get it into a good condition, I implemented it myself. I left a comment crediting you for your contribution though, I hope that's okay.

telegram-bot-api/bot.go

Lines 155 to 221 in ce4fc98

	// This code modified from the very helpful @HirbodBehnam
	// https://github.com/go-telegram-bot-api/telegram-bot-api/issues/354#issuecomment-663856473
	go func() {
	defer w.Close()
	defer m.Close()
	for field, value := range params {
	if err := m.WriteField(field, value); err != nil {
	panic(err)
	}
	}
	for _, file := range files {
	switch f := file.File.(type) {
	case string:
	fileHandle, err := os.Open(f)
	if err != nil {
	panic(err)
	}
	defer fileHandle.Close()
	part, err := m.CreateFormFile(file.Name, fileHandle.Name())
	if err != nil {
	panic(err)
	}
	io.Copy(part, fileHandle)
	case FileBytes:
	part, err := m.CreateFormFile(file.Name, f.Name)
	if err != nil {
	panic(err)
	}
	buf := bytes.NewBuffer(f.Bytes)
	io.Copy(part, buf)
	case FileReader:
	part, err := m.CreateFormFile(file.Name, f.Name)
	if err != nil {
	panic(err)
	}
	if f.Size != -1 {
	io.Copy(part, f.Reader)
	} else {
	data, err := ioutil.ReadAll(f.Reader)
	if err != nil {
	panic(err)
	}
	buf := bytes.NewBuffer(data)
	io.Copy(part, buf)
	}
	case FileURL:
	val := string(f)
	if err := m.WriteField(file.Name, val); err != nil {
	panic(err)
	}
	case FileID:
	val := string(f)
	if err := m.WriteField(file.Name, val); err != nil {
	panic(err)
	}
	default:
	panic(errors.New(ErrBadFileType))
	}
	}
	}()

[HirbodBehnam]

Hello again
That's 100% fine with me!
I'm glad I helped