WordPress file_get_contents returns empty

After submission of the contact form I hook in on the submission. The file, uploaded on the contact form, need to be send to a SOAP service. In order to receive the filedata I do the following:

Returns empty:

$base64string = base64_encode(file_get_contents($_FILES["cv"]["tmp_name"]));

My $_FILES is not empty, so the file is in place. file_get_contents **always** returns an empty string. also, allow_url_fopen is on “On”

When I test this on a single PHP file (no WordPress) it returns the BASE_64 string so it has to do something with WordPress:

Working code:

<form action="" method="POST" enctype="multipart/form-data">
		<input type="file" name="cv">
		<input type="submit" name="formsubmit">
	</form>
	<?

	if(isset($_POST['formsubmit'])){
		print_r(base64_encode(file_get_contents($_FILES["cv"]["tmp_name"])));
	}

Any ideas?