Maybe this will fix things

broker/telegram.go

@@ -79,7 +79,7 @@ func (t Telegram) SendPhoto(data tg.ClientPhotoData) {
 	// Write file into multipart buffer
 	body := new(bytes.Buffer)
 	writer := multipart.NewWriter(body)
-	part, err := writer.CreateFormFile("photo", "meme.jpg")
+	part, err := writer.CreateFormFile("photo", data.Filename)
 	if checkerr("SendPhoto/multipart.CreateFormFile", err) {
 		return
 	}
@@ -104,6 +104,8 @@ func (t Telegram) SendPhoto(data tg.ClientPhotoData) {
 		return
 	}
 
+	req.Header.Add("Content-Type", writer.FormDataContentType())
+
 	resp, err := client.Do(req)
 	checkerr("SendPhoto/http.Do", err)
 	defer resp.Body.Close()

mods/memegen.go

@@ -167,7 +167,7 @@ func memegen(broker *tg.Broker, update tg.APIMessage) {
 					broker.SendTextMessage(update.Chat, "<b>ERRORE!</b> @hamcha controlla la console!", &update.MessageID)
 					return
 				}
-				broker.SendPhoto(update.Chat, buf.Bytes(), nil, &update.MessageID)
+				broker.SendPhoto(update.Chat, buf.Bytes(), "meme.jpg", nil, &update.MessageID)
 			})
 		}
 	}

tg/broker.go

@@ -50,11 +50,12 @@ func (b *Broker) SendTextMessage(chat *APIChat, text string, original *int) {
 
 // SendPhoto sends a photo with an optional caption to a chat.
 // A reply_to message ID can be specified as optional parameter.
-func (b *Broker) SendPhoto(chat *APIChat, data []byte, caption *string, original *int) {
+func (b *Broker) SendPhoto(chat *APIChat, data []byte, filename string, caption *string, original *int) {
 	b.sendCmd(ClientCommand{
 		Type: CmdSendPhoto,
 		PhotoData: &ClientPhotoData{
 			ChatID:   chat.ChatID,
+			Filename: filename,
 			Bytes:    base64.StdEncoding.EncodeToString(data),
 			Caption:  caption,
 			ReplyID:  original,

tg/command.go

@@ -48,6 +48,7 @@ type ClientTextMessageData struct {
 type ClientPhotoData struct {
 	ChatID   int
 	Bytes    string
+	Filename string
 	Caption  *string `json:",omitempty"`
 	ReplyID  *int    `json:",omitempty"`
 }