fix: manually parse the content disposition to preserve directories

fixes ipfs/kubo#8445

multipartfile.go

@@ -110,7 +110,12 @@ func (w *multipartWalker) nextFile() (Node, error) {
 
 // fileName returns a normalized filename from a part.
 func fileName(part *multipart.Part) string {
-	filename := part.FileName()
+	v := part.Header.Get("Content-Disposition")
+	_, params, err := mime.ParseMediaType(v)
+	if err != nil {
+		return ""
+	}
+	filename := params["filename"]
 	if escaped, err := url.QueryUnescape(filename); err == nil {
 		filename = escaped
 	} // if there is a unescape error, just treat the name as unescaped