[IMP] attachment_zipped_download: provide ir.attachment.action_download mixin

This helps to download multiple attachments from any models.

attachment_zipped_download/readme/USAGE.rst

@@ -1,2 +1,78 @@
 #. Go to *Settings > Technical > Database Structure > Attachments* and select some files.
 #. Go to *Actions > Download* and a zip file containing the selected files will be downloaded.
+
+## For developer
+
+You can reuse the `IrAttachmentActionDownloadMixin` on your
+favorite models::
+
+    from odoo import models
+
+
+    class StockPicking(models.Model):
+        _name = "stock.picking"
+        _inherit = ["stock.picking", "ir.attachment.action_download"]
+
+
+Then you can add an action button on list view line or on the action button
+(when multiple lines are selected) to download all files::
+
+    <odoo>
+        <!--
+            add a button on list view to download all attachement present
+            on the given transfert
+        -->
+        <record id="vpicktree" model="ir.ui.view">
+            <field name="inherit_id" ref="stock.vpicktree"/>
+            <field name="name">stock.picking.tree download attachments</field>
+            <field name="model">stock.picking</field>
+            <field name="arch" type="xml">
+                <field name="json_popover" position="after">
+                    <button name="action_download_attachments"
+                        type="object"
+                        icon="fa-download"
+                        string="Download attachment(s)"
+                        aria-label="Download Proof documents"
+                        class="float-right"/>
+                </field>
+            </field>
+        </record>
+
+        <!--
+            Add "Download attachments" item in the Action menu when
+            multiple records are selected
+        -->
+        <record id="action_download_picking_attachements" model="ir.actions.server">
+            <field name="name">Download attachments</field>
+            <field name="model_id" ref="stock.model_stock_picking"/>
+            <field name="binding_model_id" ref="stock.model_stock_picking"/>
+            <field name="binding_view_types">list</field>
+            <field name="state">code</field>
+            <field name="code">
+                action = records.action_download_attachments()
+            </field>
+        </record>
+    </odoo>
+
+
+.. note::
+
+    Even you will be able to generate a zip file with multiple document with the
+    same name it's advice to overwrite `_compute_zip_file_name` to improve the
+    name. When a slash (`/`) is present in the path it will create a directory.
+    This example will create a directory per stock.picking using its name::
+
+        class IrAttachment(models.Model):
+            _inherit = "ir.attachment"
+
+            def _compute_zip_file_name(self):
+                self.ensure_one()
+                if self.res_model and self.res_model == "stock.picking":
+                    return (
+                        self.env[self.res_model]
+                        .browse(self.res_id)
+                        .display_name.replace("/", "-")
+                        + "/"
+                        + self.name
+                    )
+                return super()._compute_zip_file_name()