You can create a Smart Button on the Opportunity if you need to manage more Attachments than would reasonably work in the Chatter.

Activate Developer Mode:

1. Create a Custom View to show attachments in a list and allow them to be created from the list:

<tree string="Attachments" editable="top" >
  <field name="datas" widget="download_link" filename="name" string="File"/>
  <field name="name" invisible="1"/>
  <field name="company_id" groups="base.group_multi_company"/>
  <field name="create_uid"/>
  <field name="create_date"/>
</tree>

2. Create a Window Action to open attachments:

Note the Domain Value and Context Value field are both specific to the Document Model you want to find attachments for, in this case crm.lead.

[('res_id','=',active_id) ,  ('res_model','=','crm.lead')]

{'default_res_id': active_id, 'default_res_model': 'crm.lead'}

At the bottom of this Window Action, link it to the View you created in the first step:

Take note of the ID of this Window Action (found in the URL) as you will need it in the next step:

3. Create the Smart Button in your own Custom View on top of Opportunity.

Note that the Name of the button will the the ID of the Window Action you created.

<xpath expr="//button[@name='action_schedule_meeting']" position="after">
  <button class="oe_stat_button" icon="fa-file-o" type="action" name="475" string="Attached"/>
</xpath>

Your Opportunity Form View will now look like this:

When users click, they will see:

When they click CREATE they will be able to upload a new Attachment inline: