how to solve File upload validation problem in yii2 - yii2

Question

I'm trying to upload a file in yii2, the same code was working fine before but for some reason i can't figure out it stopped working after a lot of checks i notice the validate method of upload model is returning false and in the error message it says Array ( [file] => Array ( [0] => Only files with these extensions are allowed: png, jpg,jpeg,gif,pdf. ) ) but the most weird thing is i uploaded a jpg file i also try to upload a png file the same error, when i remove the check for extension in the model rules it works fineor totally remove the validation is also work, i don't know what am missing here any help would be appreciated.
NOTE //with the validation or the extension check for extension in place the codes in the if()
statement fails to execute but rather the else statement execute, removing the validation or the
extension check the code in if() works fine
class Upload extends Model
{
public $file;
public $randomCharacter;
public $fileDirectory;
public function rules()
{
return[
[['file'], 'file', 'skipOnEmpty' => false,'maxSize'=>1024 * 1024 * 2, 'extensions'=> ['png, jpg,jpeg,gif,pdf']],
];
}
public function upload($uploadPath=NULL)
{
if(isset($uploadPath) && $uploadPath !==NULL){
$filePath = $uploadPath;
}else{
$filePath = "#frontend/web/uploads/";
}
//generate random filename
$rand = Yii::$app->security->generateRandomString(10). '_' . time();
//assign generated file name to randomCharacter property
$this->randomCharacter = $rand;
//define the upload path;
if ($this->validate()){
$path = \Yii::getAlias($filePath);
$this->fileDirectory = $this->randomCharacter .'.'.$this->file->extension;
echo $path.$this->fileDirectory;
exit;
$this->file->saveAs($path.$this->fileDirectory );
return true;
}else{
// return false;
//with validation in place the else statement in executed
print_r($this->getErrors());
exit;
}
}
}

Answer 1

Remove the straight brackets of attribute "extensions"
Instead of:
[['file'], 'file', 'skipOnEmpty' => false,'maxSize'=>1024 * 1024 * 2, 'extensions'=> ['png, jpg,jpeg,gif,pdf']],
You should have this:
[['file'], 'file', 'skipOnEmpty' => false,'maxSize'=>1024 * 1024 * 2, 'extensions'=> 'png, jpg, jpeg, gif, pdf'],