I think that the problem is in the use of WP_SITEURL instead of WP_HOME for url creation.
In your case seems that the site is something.it (WP_SITEURL) but the installation directory is something.it/wordpress (WP_HOME).
Changing the urls from something.it/wp-content/uploads/gpx/la_panoramica_sponda_val_biandino.gpx
to something.it/wordpress/wp-content/uploads/gpx/la_panoramica_sponda_val_biandino.gpx
seems to solve.
@devfarm.it can it be changed or a setting can be added for this special case when the wp-content is in a different folder than the siteurl? Thank you.