The 'file name' can be totally different from the 'title' and 'file name' also contains the extension, example: 'dfdf5434.pdf' vs 'Testfile One'.
I tested your suggested {file_title} for non logged-in user it indeed works, but it gets confusing if it works for some users and not for some others.
Looking in the Code:
In content plugin file (/plugins/content/jdownloads.jdownloads.php) there already seems to be an undocumented {file_title_only} but is unclear to me when this is used, editing or even deleting the file did not have any effect.
Digging a bit further I found the related files, To get the tag working at the following views needs the following lines:
/components/com_jdownloads/views/category/tmpl/default.php
newline 1505:
$html_file = str_replace('{file_title_only}',$files[$i]->title, $html_file);
/components/com_jdownloads/views/downloadS/tmpl/default.php
newline 1130:
$html_file = str_replace('{file_title_only}',$items[$i]->title, $html_file);
/components/com_jdownloads/views/download/tmpl/default.php
newline 448:
$body = str_replace('{file_title_only}', $item->title, $body);