Add images and binary content on gist preview (#615)

internal/render/highlight.go

@@ -27,8 +27,9 @@ func (r HighlightedFile) InternalType() string {
 
 type RenderedGist struct {
 	*db.Gist
-	Lines []string
-	HTML  string
+	Lines           []string
+	HTML            string
+	PreviewMimeType *git.MimeType
 }
 
 func highlightFile(file *git.File) (HighlightedFile, error) {
@@ -76,6 +77,18 @@ func HighlightGistPreview(gist *db.Gist) (RenderedGist, error) {
 		Gist: gist,
 	}
 
+	if gist.PreviewMimeType != "" {
+		mt := &git.MimeType{ContentType: gist.PreviewMimeType}
+		if mt.CanBeEmbedded() {
+			rendered.PreviewMimeType = mt
+			return rendered, nil
+		}
+	}
+
+	if gist.Preview == "" {
+		return rendered, nil
+	}
+
 	style := newStyle()
 	lexer := newLexer(gist.PreviewFilename)
 	if lexer.Config().Name == "markdown" {