// uid of user that to add the file.
$uid = $UID;

// Final file name
$filename = $FILENAME;

// Fieldname where to attach the file
$fieldname = $FIELDNAME;

// File temporary path.
$file_tmp_path = $FILE_TMP_PATH;

// Let's go!

// Load user object.
$account = user_load($uid);

// Gather file info.
$filesize  = filesize($file_tmp_path);
$mime_type = mime_content_type($file_tmp_path);

// Create file object.
$file = new stdClass;
$file->uid      = $uid;
$file->filename = <desired filename>;
$file->uri      = $file_tmp_path;
$file->filemime = $mime_type;
$file->filesize = $filesize;
$file->status   = FILE_STATUS_PERMANENT;

$destination .= '/' . $filename;

// Move to its final destination and add it to the database.
// file_move moves the file and is in charge of add the needed DB records.
$file = file_move($file, $destination, FILE_EXISTS_RENAME);

// Attach file to user.
$account->{$fieldname}[LANGUAGE_NONE][0]['fid'] = $file->fid;


$user_save($account);