[SalesForce] Attachments added to Task not displaying in Attachments related List in Leads

You just need to Edit the task itself to see the Add Attachment button. It's only available in the task edit view.

It's a bit of a hack, but you could create a Visualforce page and controller action that will redirect the user to the most recently created Task for a record.

Set the &retURL going into the task creation to send the user to this new VF page and include the parent opporutnity record ID in the query string.

/00T/e?what_id={!Opportunity.Id}&retURL=%2Fapex%2FredirectLastTask?rId= {!Opportunity.Id}

public with sharing class redirectTaskController {
    string recordId = '';

    public redirectTaskController() {
        recordId = ApexPages.currentPage().getParameters().get('rId');
    }

    public PageReference redirectToTask() {
        PageReference activityPage = new PageReference('/' + recordId);

        Task[] recentTasks = [SELECT Id, CreatedDate 
                              FROM Task WHERE WhatId = :recordId 
                              ORDER BY CreatedDate DESC LIMIT 1];

        // Check that the new Task if fairly recent.
        if (recentTasks.Size() == 1 && 
                recentTasks[0].CreatedDate.addHours(1) > Datetime.now()) {
            activityPage = new PageReference('/' + recentTasks[0].Id);
        }

        activityPage.setRedirect(true);
        return activityPage;
    }
}

Visualforce Page

<apex:page controller="redirectTaskController" action="{!redirectToTask}" >
    <p>Redirecting to task</p>
</apex:page>

Another idea would be to create a Visualforce page that uses the Task standardController and provides a custom save action.