Update upload.php

It seems logical to add this line, since such verification exists for $storeFolder:
```php
strpos($storeFolderThumb,'./',strlen($thumbs_base_path))!==FALSE
```

Also replaced ```php $_POST['path_thumb'] ``` with descriptive name, and removed that ```php +$thumb_pos ``` and ```php +$path_pos ``` because at this lines we are already sure both values equal to 0.

Besides that, I'm not sure if this code does anything useful:
```php
    else{
	    move_uploaded_file($tempFile,$targetFile);
	    chmod($targetFile, 0755);
	}
```

because we are already moved uploaded file at the beginning (lines 60-61)