Actually that is pretty small compared to many sites that make files that are several megs ...

Try making a New Product and then add the Attribute to that New Product with the file ...

Does anything change?

Something is not right here and really doesn't make a lot of sense that the file is not being recognized ...

NOTE: I am assuming that when you loaded the file, via FTP to the /download directory that you first deleted any existing files of the same name from the server ...