account doesn't exist in your trigger, therefore when you code "account.name" that means the field "name" assocaited with the sobject type account, not the name of an instance of account.

You'll need to retrieve the account details through SOQL.  I'd suggest you do this up front to ensure you won't blow governor limits in the event of a bulk upload.

Something like the following should do it:

trigger Send_email on Attachment (after insert) {
   List<Id> accIds=new List<Id>();
   for (Attachment att : trigger.new)
   {
      accIds.add(att.parentId);
   }
   Map<Id, Account> accounts=new Map<Id, Account>();
   accounts.putAll([select id, Name from Account where id IN :accIds]);
   Attachment attach = trigger.new[0];
   Messaging.SingleEmailMessage mail = new    Messaging.SingleEmailMessage();
   String[] toAddresses = new String[] {'ja@example.com'};
   mail.setToAddresses(toAddresses);
   mail.setReplyTo('support@example.com');
   mail.setSenderDisplayName('Salesforce Support');
   Account acc=accounts.get(attach.parentId);
   if (null!=acc)
   {
      mail.setSubject('Attachment Added : ' + acc.name);
   }
   mail.setPlainTextBody('work please');
   Messaging.sendEmail(new Messaging.SingleEmailMessage[] { mail });
}