Google Apps Script can't export anything else than pdf natively but the document API does so you can use urlFetch with parameters to get what you want (don't change these parameters, the request is anonymous and can stay so).
Here is the code, it doesn't require too much explanations except that the urlFetch must be authorized (the process is not the usual one, you'll get a second authorization for this script.)
function emailDocTestasDocx() {
var id = '1I9KIVTLieQbNnmz09zfOBSBNwZ9Tp7B0kfpysaf-ooY';// an example of Google doc
var url = 'https://docs.google.com/feeds/';
var doc = UrlFetchApp.fetch(url+'download/documents/Export?exportFormat=doc&format=doc&id='+id,
googleOAuth_('docs',url)).getBlob();
var me = Session.getEffectiveUser().getEmail();
MailApp.sendEmail(me, 'test', 'see attachment', {attachments:[doc]});
}
function googleOAuth_(name,scope) {
var oAuthConfig = UrlFetchApp.addOAuthService(name);
oAuthConfig.setRequestTokenUrl("https://www.google.com/accounts/OAuthGetRequestToken?scope="+scope);
oAuthConfig.setAuthorizationUrl("https://www.google.com/accounts/OAuthAuthorizeToken");
oAuthConfig.setAccessTokenUrl("https://www.google.com/accounts/OAuthGetAccessToken");
oAuthConfig.setConsumerKey('anonymous');
oAuthConfig.setConsumerSecret('anonymous');
return {oAuthServiceName:name, oAuthUseToken:"always"};
}