It seems that the standard uploader does not check the allowed filesize. Can I check the filesize and avoid that it is getting stored on the server?

Yes, please copy the following code into the Begin Submit Piece:

// get the name of the uploaded file element
$filename = ff_getSubmit('upload');
// check size of uploaded file
if ($filename && filesize($filename) >= 1048576)
     // 1MB
     // set a custom error code
     $this->status = _FF_STATUS_UPLOAD_FAILED;
     // delete uploaded file
     header("Location: URLtoForm&error=true");

Change URL to Form to the URL that links to the first page of your form. Don't change '&error=true'

Copy this code into the Before Form Piece of the Form:

$show_error = JRequest::getVar('error');
if ($show_error == 'true')
     echo '<script type="text/javascript">alert("Error_Message");</script>';

Change 'Error_Message' to your desired error message.

Now the form should redirect the user to the first page of the form and show a PopUp message.

Note that this does not work for forms in IFrames!

Beside this PHP solution there are many workarounds in the internet which only use Javascript.

Please check for more information.

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