Enable this plugin to connect <input type="file" name="fields[yourField]" /> as asset to yourField

AssetUploadPlugin.php

<?php
namespace Craft;

class AssetUploadPlugin extends BasePlugin
{
    function getName()
    {
        return Craft::t('Upload assets on the front-end');
    }
    
    function getVersion()
    {
        return '2.0';
    }
    
    function getDeveloper()
    {
        return 'Bob Olde Hampsink';
    }
    
    function getDeveloperUrl()
    {
        return 'http://www.itmundi.nl';
    }
    
    function init() {
    
        // Only works on the front-end
        if(craft()->request->isSiteRequest()) {
        
            // Trigger when an entry is saved
            craft()->on('entries.onSaveEntry', function(Event $event) {
                
                craft()->assetUpload->process(ElementType::Entry, $event->params['entry']);
                            
            });
            
            // Trigger when a user profile is saved
            craft()->on('users.onSaveProfile', function(Event $event) {
                       
                craft()->assetUpload->process(ElementType::User, $event->params['user']);
            
            });
        
        }
    
    }

}

AssetUploadService.php

<?php
namespace Craft;

class AssetUploadService extends BaseApplicationComponent
{
    
    static private $inEvent = false;

    public function process($type, $entry) {
    
        // Only trigger when files are attached in the initial post
        if(isset($_FILES) && !self::$inEvent) {
            
            // Make sure saveEntry event only gets fired once
            self::$inEvent = true;
            
            // Loop trough filefields
            foreach($_FILES as $files) {
            
                if(is_array($files['name'])) {
                          
                    foreach($files['name'] as $index => $file) {
                    
                        // Make sure it's always an array
                        if(!is_array($file)) {
                            $file = array($file);
                        }
                        
                        // Can have multiple files, loop trough
                        foreach($file as $key => $item) {
                        
                            // Find in which upload source the file belongs
                            // TODO: multiple upload sources
                            $folderId = end(explode(':', craft()->fields->getFieldByHandle($index)->settings['sources'][0]));
                            $sourceId = craft()->assets->getFolderById($folderId)->sourceId;
                            
                            // Find the path to the upload source
                            $uploadDir = craft()->assetSources->getSourceById($sourceId)->settings['path'];
                            
                            // Rename file if necessary - use the same conflict resolution Craft uses
                            $i = 1;
                            $newpieces = explode(".", $item);
                            $frontpath = str_replace('.'.end($newpieces),'',$item);
                            while(file_exists($uploadDir.$item)) {
                                $item = $frontpath.'_'.$i.'.'.end($newpieces);
                                $i++;
                            }
                            
                            // move file from temp to assets dir
                            if(move_uploaded_file($files['tmp_name'][$index][$key], $uploadDir.$item)){
                                
                                // Create new AssetFile Model
                                $asset = new AssetFileModel();
                                $asset->sourceId = $sourceId;
                                $asset->folderId = $folderId;
                                $asset->filename = IOHelper::getFileName($item);
                                $asset->kind = IOHelper::getFileKind(IOHelper::getExtension($item));
                                $asset->size = filesize($uploadDir.$item);
                                $asset->dateModified = IOHelper::getLastTimeModified($uploadDir.$item);
                                
                                if ($asset->kind == 'image')
                                {
                                    list ($width, $height) = getimagesize($uploadDir.$item);
                                    $asset->width = $width;
                                    $asset->height = $height;
                                }
                                
                                // Index file
                                craft()->assets->storeFile($asset);

                                // Update entry with asset
                                $entry->getContent()->setAttributes(array(
                                    $index => array(@$asset->id)
                                ));
                                
                                // If entry
                                if($type == ElementType::Entry) {
                                
                                    // Save entry
                                    craft()->entries->saveEntry($entry);
                                    
                                // If user
                                } elseif($type == ElementType::User) {
                                
                                    // Save profile
                                    craft()->users->saveProfile($entry);
                                    
                                }

                            }
                            
                        }
                    
                    }
                    
                }
            
            }
        
        }
    
    }
    
}