For running J!5.1 you must https://fabrikar.com/forums/index.php?wiki/update-from-github/ or include the new file manually https://fabrikar.com/forums/index.php?threads/joomla-5-1-and-fabrik-cannot-find-files-error.54473/post-285151 See also Announcements
Please see our announcement here.
global $_FILES;
// get information about the uploaded file whose full element name is
// 'tablename___elementname';
$fileinfo = $_FILES['tablename___elementname'];
// insert its file type and name into your hidden fields
JRequest::setVar('tablename___filetype', $fileinfo['type'], 'post');
JRequest::setVar('tablename___filename', $fileinfo['[FONT=monospace]name[/FONT]'], 'post');