In a managed_file upload field, how do I prevent an overflow with long file names?
I would suggest adding a class to the fieldset and adding a class to the css file of your theme.

Add something like this to the fieldset.

'#attributes' =&gt; array('class' =&gt; array('my-fieldset'));

And then something like this to your css file

.my-fieldset {max-width: 100%;}