MM-23185 - Markdown image hosted by plugins are not shown if l… (#14185)

* avoid image proxy for local images

* added test for local images

api4/image.go

@@ -5,6 +5,9 @@ package api4
 
 import (
 	"net/http"
+	"net/url"
+
+	"github.com/mattermost/mattermost-server/v5/model"
 )
 
 func (api *API) InitImage() {
@@ -12,11 +15,17 @@ func (api *API) InitImage() {
 }
 
 func getImage(c *Context, w http.ResponseWriter, r *http.Request) {
-	url := r.URL.Query().Get("url")
+	actualURL := r.URL.Query().Get("url")
+	parsedURL, err := url.Parse(actualURL)
+	if err != nil {
+		c.Err = model.NewAppError("getImage", "api.image.get.app_error", nil, err.Error(), http.StatusBadRequest)
+		return
+	}
 
-	if *c.App.Config().ImageProxySettings.Enable {
-		c.App.ImageProxy().GetImage(w, r, url)
+	// in case image proxy is enabled and we are fetching a remote image (NOT static or served by plugins), pass request to proxy
+	if *c.App.Config().ImageProxySettings.Enable && parsedURL.IsAbs() {
+		c.App.ImageProxy().GetImage(w, r, actualURL)
 	} else {
-		http.Redirect(w, r, url, http.StatusFound)
+		http.Redirect(w, r, actualURL, http.StatusFound)
 	}
 }