[MM-50827] Detect content-type if missing in response during link metadata generation (#22885)

* Detect content-type if missing in response during link metadata generation

* Initialize buffer only when necessary

server/channels/app/post_metadata.go

@@ -4,6 +4,7 @@
 package app
 
 import (
+	"bufio"
 	"bytes"
 	"fmt"
 	"image"
@@ -764,7 +765,24 @@ func cacheLinkMetadata(requestURL string, timestamp int64, og *opengraph.OpenGra
 	platform.LinkCache().SetWithExpiry(strconv.FormatInt(model.GenerateLinkMetadataHash(requestURL, timestamp), 16), metadata, platform.LinkCacheDuration)
 }
 
+// peekContentType peeks at the first 512 bytes of p, and attempts to detect
+// the content type.  Returns empty string if error occurs.
+func peekContentType(p *bufio.Reader) string {
+	byt, err := p.Peek(512)
+	if err != nil && err != bufio.ErrBufferFull && err != io.EOF {
+		return ""
+	}
+	return http.DetectContentType(byt)
+}
+
 func (a *App) parseLinkMetadata(requestURL string, body io.Reader, contentType string) (*opengraph.OpenGraph, *model.PostImage, error) {
+	if contentType == "" {
+		bufRd := bufio.NewReader(body)
+		// If the content-type is missing we try to detect it from the actual data.
+		contentType = peekContentType(bufRd)
+		body = bufRd
+	}
+
 	if contentType == "image/svg+xml" {
 		image := &model.PostImage{
 			Format: "svg",

server/channels/app/post_metadata_test.go

@@ -2595,6 +2595,18 @@ func TestParseLinkMetadata(t *testing.T) {
 		}, dimensions)
 	})
 
+	t.Run("image with no content-type given", func(t *testing.T) {
+		og, dimensions, err := th.App.parseLinkMetadata(imageURL, makeImageReader(), "")
+		assert.NoError(t, err)
+
+		assert.Nil(t, og)
+		assert.Equal(t, &model.PostImage{
+			Format: "png",
+			Width:  408,
+			Height: 336,
+		}, dimensions)
+	})
+
 	t.Run("malformed image", func(t *testing.T) {
 		og, dimensions, err := th.App.parseLinkMetadata(imageURL, makeOpenGraphReader(), "image/png")
 		assert.Error(t, err)