Looking into /contact-form-7/admin/wpcf7-admin.js file, there is:
var dropdown_icon = jQuery('<img src="../wp-content/plugins/contact-form-7/images/dropdown.gif" />');
which doesn't work if having a different path for plugins directory. I had to manually modify to:
var dropdown_icon = jQuery('<img src="../plugins/contact-form-7/images/dropdown.gif" />');
in order to work for me, as I have website.tld/plugins/ instead of website.tld/wp-content/plugins/. Anyway to correct this so I wouldn't have to modify the code everytime I update Contact Form 7 ?