wp_handle_upload always returns empty

I’m trying to set up a file upload in a plugin, but for some reason, wp_handle_upload always returns the following error:

File is empty. Please upload something more substantial. This error could also be caused by uploads being disabled in your php.ini or by post_max_size being defined as smaller than upload_max_filesize in php.ini.

I checked post_max_size and upload_max_filesize and they are both set to 64M, much bigger than the files I’m trying to upload.

Here is the code I’m using to handle the upload:

$newpath = 'uploads';
update_option('upload_path',$newpath);
$overrides = array( 'test_form' => false);
$cpnfile = wp_handle_upload($_FILES['cpnfile'], $overrides);
print_r($cpnfile);

Any help would be much appreciated, thanks!