I had the same dilemma, and may have just figured out a duct-tape-like solution:
Open the file /plugins/Contribution/views/public/contribution/type-form.php
Line 17 has:
<?php echo $this->formFile('contributed_file', array('class' => 'fileinput')); ?>
Comment it out, so it looks like this:
<?php #echo $this->formFile('contributed_file', array('class' => 'fileinput')); ?>
…and right underneath it, add:
<input type="file" name="contributed_file[]" id="contributed_file" class="fileinput" multiple>
Seems to work?
Jeremy