Hello,
it seems that links for downloading files in fly views don't work.
At the fly layout edition view, in the field properties I set "Create a link" to true and select Download in Task/Action combo. After building the component, once the file is uploaded in form view, it cannot be downloaded in fly view, a plain text filename is displayed instead of a link.

Simple workaround is to replace generated JHTML for the field in fly template file with a JDOM call:
<?php echo JDom::_('html.fly.file', array(
'dataKey' => 'attachment',
'dataObject' => $this->item,
'height' => 'auto',
'indirect' => true,
'root' => '[DIR_SOME_PATH]',
'target' => 'download'
));?>

Fixed.