<form action="" method="post" enctype="multipart/form-data">
<input type="hidden" name="mod" value="materials"/>
<input type="hidden" name="action" value="doadd"/>
...
<div class="form-group">
<label for="img">Выберете картинку:</label>
<input class="btn btn-default" type="file" name="img" id="img"/>
</div>
<div class="form-group">
<label for="drawings">Выберете чертежи (архив):</label>
<input class="btn btn-default" type="file" name="drawings" id="drawings"/>
</div>
<div class="form-group">
<input class="btn btn-success" type="submit" value="Добавить"/>
</div>
</form>