fix computation of image ids

Signed-off-by: Tobias Erbshäußer <tobias@tesoft.dev>

backend/blog.go

@@ -66,7 +66,7 @@ func ParseArticle(reader io.Reader, filePrefix string) (*Article, error) {
 	}
 
 	usedFiles := make(map[string]ArticleFile)
-	nextFileId := int64(0)
+	prevFileId := int64(0)
 
 	pc, html, err := md.Parse(func(path string) (string, error) {
 		content, ok := tarFiles[strings.TrimPrefix(path, "./")]
@@ -74,10 +74,10 @@ func ParseArticle(reader io.Reader, filePrefix string) (*Article, error) {
 			return "", errors.New("file '" + path + "' not found")
 		}
 
-		usedFiles[path] = ArticleFile{nextFileId, content}
-		nextFileId++
+		prevFileId++
+		usedFiles[path] = ArticleFile{prevFileId, content}
 
-		return filePrefix + strconv.FormatInt(nextFileId, 10), nil
+		return filePrefix + strconv.FormatInt(prevFileId, 10), nil
 	}, readmeBytes)
 	if err != nil {
 		return nil, err