Add /v4/image api (#8230)

* add image api

* i suppose i should add a test...

* only redirect to image proxy

api4/image_test.go

@@ -0,0 +1,52 @@
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package api4
+
+import (
+	"net/http"
+	"net/url"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/mattermost/mattermost-server/model"
+)
+
+func TestGetImage(t *testing.T) {
+	th := Setup().InitBasic()
+	defer th.TearDown()
+
+	th.Client.HttpClient.CheckRedirect = func(*http.Request, []*http.Request) error {
+		return http.ErrUseLastResponse
+	}
+
+	originURL := "http://foo.bar/baz.gif"
+
+	r, err := http.NewRequest("GET", th.Client.ApiUrl+"/image?url="+url.QueryEscape(originURL), nil)
+	require.NoError(t, err)
+	r.Header.Set(model.HEADER_AUTH, th.Client.AuthType+" "+th.Client.AuthToken)
+
+	th.App.UpdateConfig(func(cfg *model.Config) {
+		cfg.ServiceSettings.ImageProxyType = nil
+	})
+
+	resp, err := th.Client.HttpClient.Do(r)
+	require.NoError(t, err)
+	assert.Equal(t, http.StatusNotFound, resp.StatusCode)
+
+	th.App.UpdateConfig(func(cfg *model.Config) {
+		cfg.ServiceSettings.ImageProxyType = model.NewString("willnorris/imageproxy")
+		cfg.ServiceSettings.ImageProxyURL = model.NewString("https://proxy.foo.bar")
+	})
+
+	r, err = http.NewRequest("GET", th.Client.ApiUrl+"/image?url="+originURL, nil)
+	require.NoError(t, err)
+	r.Header.Set(model.HEADER_AUTH, th.Client.AuthType+" "+th.Client.AuthToken)
+
+	resp, err = th.Client.HttpClient.Do(r)
+	require.NoError(t, err)
+	assert.Equal(t, http.StatusFound, resp.StatusCode)
+	assert.Equal(t, "https://proxy.foo.bar//"+originURL, resp.Header.Get("Location"))
+}