How can I check if there's a file selected for upload?

[eluser]Monkeytail[/eluser]
How can I check if there's a file selected for upload?

When I send the form filled with text and an image it all works fine.
Some articles don't need an image.
When I leave the file input field empty.. the system throws an error saying.. there is nothing to upload: and that's correct

The thing I'am after is to add a new article with or without an image.