Drupal 7 form file processing

form.php

<?php

function mymodule_form($form, &$form_state) {
    // [...]
    
    $form['attachment'] = array(
        '#type' => 'file',
        '#title' => t('Attachment'),
    );
    
    // [...]
}

function mymodule_form_validate(&$form, &$form_state) {
    // [...]
    
    // If a file was attached, validate its size and remember that it exists.
    $megabytes = 25;
    $file_validators = array(
        'file_validate_size' => array(($megabytes*1024*1024)),
        "file_validate_extensions" => array("7z ai bmp doc docx eml eps gif gz jpeg jpg log pdf png pps ppsx ppt pptx psd rtf sql tif txt xls xlsx xml zip")
    );
    // Save the file as a temporary file.
    $file = file_save_upload('attachment', $file_validators);
    if ($file === FALSE) {
        form_set_error('attachment', t('Your file is either too big or using a disallowed file extension.'));
    }
    elseif ($file !== NULL) {
        $form_state['values']['attachment'] = $file;
    }
    
    // [...]
}

function mymodule_form_submit(&$form, &$form_state) {
    // [...]
    
    $has_attachment = !empty($form_state['values']['attachment']);
    
    if($has_attachment) {
        $filepath = drupal_realpath($form_state['values']['attachment']->uri);
        
        _mymodule_dosomething_with_a_file($filepath);
    }
    
    // [...]
}