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<?php

$page->of(false); 

// instantiate the class and give it the name of the HTML field 
$u = new WireUpload('your_file_field_name'); 

// tell it to only accept 1 file
$u->setMaxFiles(1); 

// tell it to rename rather than overwrite existing files
$u->setOverwrite(false); 

// have it put the files in their final destination. this should be okay since
// the WireUpload class will only create PW compatible filenames
$dest = $config->path('files').$page->id."/";
$u->setDestinationPath($dest); 

// tell it what extensions to expect
$u->setValidExtensions(['jpg', 'jpeg', 'gif', 'png']); 

// execute() returns an array, so we'll foreach() it even though only 1 file
foreach($u->execute() as $filename) $page->images->add($filename); 

// save the page
$page->save(); 

<?php

/**
 *	@param u	processwire user 
 * 
 */
public function changeProfileImage($u) {

	// upload image
	$upload = new WireUpload('imageFile');
	$upload->setMaxFiles(1);
	$upload->setMaxFileSize(50000); // 50kb
    $upload->setOverwrite(false);
    $upload->setDestinationPath($this->config->paths->files);
    $upload->setValidExtensions(['jpg', 'jpeg', 'gif', 'png']);

    // add image to the user
    $u->of(false);
	$u->img->removeAll(); // remove all existing first
	$u->save(); // save before adding new
    foreach($upload->execute() as $filename) $u->img->add($this->config->paths->files . $filename);
	$u->save();

	// delete temp image
	$files->unlink($this->config->paths->files . $filename);
		
}