So in JForm usually a field can have a filter attribute like filter="cmd" which allows JForm to filter the users input for that field. However, in the FileList & FolderList fields the filter attribute is overwritten and used in the field itself rather than by JForm.

In these fields the filter attribute is passed to JFolder::files( $path, $this->filter ). This means we can no longer filter the actual form field itself.

Is this intentional or is it an oversight?

If it is an oversight then the attribute that filters JFolder needs changing.

The fields in question are:

libraries/joomla/form/fields/filelist.php
libraries/joomla/form/fields/folderlist.php

I'm happy to create a pull request if you like, changing filter=".css" to filefilter="" perhaps?

This would be a breaking change as there are a number of Joomla forms that use this field that will need the attribute changing as well:

administrator/components/com_languages/models/forms/language.xml
     filter="\.gif$"
components/com_mailto/mailto.xml
     filter="."
plugins/editors/codemirror/codemirror.xml
     filter="\.css$"
plugins/editors/tinymce/form/setoptions.xml
     filter="\.js$"