source: http://docs.buddyforms.com/article/147-upload-files-capability-from-the-frontend-for-different-user-roles

buddyforms_allow_contributor_uploads.php

<?php
/**
 * Check if a contributor have the needed rights to upload images and add this capabilities if needed.
 */
			
add_action('init', 'buddyforms_allow_contributor_uploads');
			
function buddyforms_allow_contributor_uploads() {
    if ( current_user_can('contributor') && !current_user_can('upload_files') ){
        $contributor = get_role('contributor');
        $contributor->add_cap('upload_files');
    }
}