diff --git a/contented.go b/contented.go
index f647d5c..3708e66 100644
--- a/contented.go
+++ b/contented.go
@@ -10,6 +10,7 @@ import (
 	"mime"
 	"mime/multipart"
 	"net/http"
+	"net/textproto"
 
 	telegram "github.com/go-telegram-bot-api/telegram-bot-api"
 )
@@ -95,7 +96,11 @@ func (this *NTFServer) ContentedUploadSync(fileId string, expectSizeBytes int64)
 	uploadBody := bytes.Buffer{}
 	formValues := multipart.NewWriter(&uploadBody)
 
-	filePart, err := formValues.CreateFormFile("f", fileName)
+	// Adapted from (mime/multipart)Writer.CreateFormFile to support a custom interior content type
+	multipartFileHeader := make(textproto.MIMEHeader)
+	multipartFileHeader.Set("Content-Disposition", fmt.Sprintf(`form-data; name="%s"; filename="%s"`, "f", fileName))
+	multipartFileHeader.Set("Content-Type", fileContentType)
+	filePart, err := formValues.CreatePart(multipartFileHeader)
 	if err != nil {
 		return "", err
 	}
@@ -116,7 +121,7 @@ func (this *NTFServer) ContentedUploadSync(fileId string, expectSizeBytes int64)
 	if err != nil {
 		return "", err
 	}
-	req.Header.Set("Content-Type", fileContentType)
+	req.Header.Set("Content-Type", `multipart/form-data`)
 	uploadResp, err := http.DefaultClient.Do(req)
 	if err != nil {
 		return "", fmt.Errorf("Upload failed: %s", err.Error())