Demo for https://processwire.com/talk/topic/2089-create-simple-forms-using-api/?p=106660

pw-form-api-file-upload.php

<?php

// Init vars
$out = '';

// Temp upload path for file uploads
$upload_path = $config->uploadTmpDir;

// create form
$form = $modules->get("InputfieldForm");
$form->action = "./";
$form->method = "post";
$form->attr("enctype", "multipart/form-data");
$form->attr("id+name",'operating-expenses-itemization-form');


$field = $modules->get("InputfieldMarkup");
$field->markupText = "<h4>Belege hochladen</h4><p>Bitte reichen Sie Kopien der Darlehensauszüge
ein.</p><p>Erlaubte Dateiformate: <b>JPG</b> / <b>PNG</b> / <b>PDF</b><br />Maximale Dateigröße:
<b>2 mb</b> (pro Datei)</p>";
$fieldsetFinanzierungPrivat->append($field);

$field = $modules->get("InputfieldFile");
$field->skipLabel = Inputfield::skipLabelBlank;
$field->description = "";
$field->required = 0;
$field->attr("name+id", 'private_expenses_documents_1');
$field->destinationPath = $upload_path;
$field->extensions = "jpg jpeg png pdf";
$field->maxFiles = 1;
$field->maxFilesize = 2*1024*1024; // ~2mb
$fieldsetFinanzierungPrivat->append($field);

$field = $modules->get("InputfieldFile");
$field->skipLabel = Inputfield::skipLabelBlank;
$field->description = "";
$field->required = 0;
$field->attr("name+id", 'private_expenses_documents_2');
$field->destinationPath = $upload_path;
$field->extensions = "jpg jpeg png pdf";
$field->maxFiles = 1;
$field->maxFilesize = 2*1024*1024; // ~2mb
$fieldsetFinanzierungPrivat->append($field);

$form->append($fieldsetFinanzierungPrivat);


$fieldsetBenachtigungsart = $modules->get('InputfieldFieldset');
$fieldsetBenachtigungsart->label = 'Bitte bestätigen Sie folgende Angaben';

$field = $modules->get("InputfieldCheckbox");
$field->label = "Vollständigkeitserklärung";
$field->label2 = " Die Sonderbetriebsausgaben des Vorjahres wurden hiermit vollständig erklärt.";
$field->attr('id+name','confirmation_tax');
$field->required = 1;
$fieldsetBenachtigungsart->append($field);

$form->append($fieldsetBenachtigungsart);


$submit = $modules->get("InputfieldSubmit");
$submit->skipLabel = Inputfield::skipLabelBlank;
$submit->attr("value","» Sonderbetriebsausgaben verbindlich einreichen");
$submit->attr("id+name","submit");
$form->append($submit);



// form was submitted so we process the form
if($input->post->submit) {

  // user submitted the form, process it and check for errors
  $form->processInput($input->post);

  // Workaround to get errors from the WireUpload used by the file field
  // Refering to https://gist.github.com/somatonic/5236008
  if($form->get("private_expenses_documents_1")->getWireUpload()->getErrors()){
    $errors = $form->get("private_expenses_documents_1")->getWireUpload()->getErrors();
    foreach($errors as $error) $form->get("private_expenses_documents_1")->error($error);
  }

  if($form->get("private_expenses_documents_2")->getWireUpload()->getErrors()){
    $errors = $form->get("private_expenses_documents_2")->getWireUpload()->getErrors();
    foreach($errors as $error) $form->get("private_expenses_documents_2")->error($error);
  }


  // File uploads
  
  // Init vars
  $files = array();
  $uploaded_documents = array();

  if (empty($files)) {

    // collect all file upload fields
    $files[] = explode("|", $form->get("private_expenses_documents_1")->value);
    $files[] = explode("|", $form->get("private_expenses_documents_2")->value);

    // Iterate throught files array and build documents array
    foreach($files as $file){
      foreach($file as $_file){
        if($_file && file_exists($upload_path . $_file)){
          $uploaded_documents[] = $upload_path . $_file;
        }
      }
    }

  }


  if($form->getErrors()) {

    $out .= "<div><h3>Das Formular enthält Fehler!</span></h3></p></div>";

    // The form is processed and populated
    // but contains errors
    $out .= $form->render();

  } else {

    // PDF Generation, send mail (with generated PDF plus
    // files as attachment (Variable $uploaded_documents))

  }
} else {
  
  // Render out form without processing
  $out .= $form->render();
}

// Ausgabe
echo $out;