feat(serverHandler/upload): restrict upload form name scope

Previously, if form name is not `dirfile` or `innerdirfile`, assume form
name is `dir` and skip checking the name provided.

Now if provided name is unrecognized, just ignore the whole form part
and will not write its content to file.

Author: marjune163

src/serverHandler/upload.go

@@ -85,6 +85,8 @@ func (h *handler) saveUploadFiles(fsPrefix string, createDir, overwriteExists bo
 			if prefixSlashIndex > 0 {
 				fsInfix = filepath[prefixSlashIndex+1:]
 			}
+		} else if formname != file {
+			continue
 		}
 
 		filePrefix := fsPrefix