form_set_error for specific file from unlimited file field

Hi there,

I`m developing a validation where I have to check all files of an unlimited files field for duplicates.
Therefore I need to highlight the files which are causing the error.

I've tried several posibilities but I can`t find the solution for this.

form_set_error("field_csv][und][$key","custom error message in german ;)");

How can I achieve that specific files are highlighted?