Not like any other post variables, file element information is posted as two dimensional array. So, getting post data for a file element can be accomplished as below.
<?php
if(isset($_POST['btnUpload'])){
$name = $_FILES['upfile']['name'];
$tmpname = $_FILES['upfile']['tmp_name'];
$sizer = $_FILES['upfile']['size'];
// open file to read data;
$fp = fopen($tmpname, 'r');
// You can do whatever you need to from this point.
// The example below reads the contents of uploaded text file.
$content = fread($fp, filesize($tmpname));
fclose($fp);
// break data from csv into array line by line
$data = preg_split('/\r\n|\r|\n/', $content);
?>
<FORM action="<?=$_SERVER['PHP_SELF']=>" method="post" enctype="multipart/form-data">
<input name="upfile" id="upfile" type="file">
<input value="Upload" name="btnUpload" id="btnUpload" type="submit">
</FORM>