[MM-25677] Content-Type is optional (#14705)

* Content-Type is optional

mime.ParseMimeType returns and "no media type" error if the passed
string is empty.

Given that the Content-Type header is optional we shouldn't return an error
in that case, so we're fixing that allowing the users to call the webhook
without passing that header

* Include webhook id in the error message

Given that the number of webhooks could be big the user could
need the id to check which one of the multiple webhooks are failing
so include the id aids in that part

web/webhook.go

@@ -28,11 +28,22 @@ func incomingWebhook(c *Context, w http.ResponseWriter, r *http.Request) {
 	r.ParseForm()
 
 	var err *model.AppError
+	var mediaType string
 	incomingWebhookPayload := &model.IncomingWebhookRequest{}
-	mediaType, _, mimeErr := mime.ParseMediaType(r.Header.Get("Content-Type"))
-	if mimeErr != nil && mimeErr != mime.ErrInvalidMediaParameter {
-		c.Err = model.NewAppError("incomingWebhook", "api.webhook.incoming.error", nil, mimeErr.Error(), http.StatusBadRequest)
-		return
+	contentType := r.Header.Get("Content-Type")
+	// Content-Type header is optional so could be empty
+	if contentType != "" {
+		var mimeErr error
+		mediaType, _, mimeErr = mime.ParseMediaType(contentType)
+		if mimeErr != nil && mimeErr != mime.ErrInvalidMediaParameter {
+			c.Err = model.NewAppError("incomingWebhook",
+				"api.webhook.incoming.error",
+				nil,
+				"webhook_id="+id+", error: "+mimeErr.Error(),
+				http.StatusBadRequest,
+			)
+			return
+		}
 	}
 
 	defer func() {
@@ -58,7 +69,12 @@ func incomingWebhook(c *Context, w http.ResponseWriter, r *http.Request) {
 		err := decoder.Decode(incomingWebhookPayload, r.PostForm)
 
 		if err != nil {
-			c.Err = model.NewAppError("incomingWebhook", "api.webhook.incoming.error", nil, err.Error(), http.StatusBadRequest)
+			c.Err = model.NewAppError("incomingWebhook",
+				"api.webhook.incoming.error",
+				nil,
+				"webhook_id="+id+", error: "+err.Error(),
+				http.StatusBadRequest,
+			)
 			return
 		}
 	} else {