diff --git a/addons/hr_expense/models/hr_expense.py b/addons/hr_expense/models/hr_expense.py
index c1ea26218b7..8b3d168af5c 100644
--- a/addons/hr_expense/models/hr_expense.py
+++ b/addons/hr_expense/models/hr_expense.py
@@ -1097,6 +1097,17 @@ class HrExpenseSheet(models.Model):
     # Mail Thread
     # --------------------------------------------
 
+    def _get_mail_thread_data_attachments(self):
+        """
+        In order to see in the sheet attachment preview the corresponding
+        expenses' attachments, the latter attachments are added to the fetched data for the sheet record.
+        """
+        self.ensure_one()
+        res = super()._get_mail_thread_data_attachments()
+        expense_ids = self.expense_line_ids
+        expense_attachments = self.env['ir.attachment'].search([('res_id', 'in', expense_ids.ids), ('res_model', '=', 'hr.expense')], order='id desc')
+        return res | expense_attachments
+
     def _track_subtype(self, init_values):
         self.ensure_one()
         if 'state' in init_values and self.state == 'approve':
diff --git a/addons/hr_expense/views/hr_expense_views.xml b/addons/hr_expense/views/hr_expense_views.xml
index 89b50d9f122..7024d47ec60 100644
--- a/addons/hr_expense/views/hr_expense_views.xml
+++ b/addons/hr_expense/views/hr_expense_views.xml
@@ -796,6 +796,7 @@
                         </page>
                      </notebook>
                 </sheet>
+                <div class="o_attachment_preview"/>
                 <div class="oe_chatter">
                     <field name="message_follower_ids"/>
                     <field name="activity_ids"/>
diff --git a/addons/mail/controllers/discuss.py b/addons/mail/controllers/discuss.py
index 7e63fa7da9e..85a09d93910 100644
--- a/addons/mail/controllers/discuss.py
+++ b/addons/mail/controllers/discuss.py
@@ -431,31 +431,8 @@ class DiscussController(http.Controller):
 
     @http.route('/mail/thread/data', methods=['POST'], type='json', auth='user')
     def mail_thread_data(self, thread_model, thread_id, request_list, **kwargs):
-        res = {'hasWriteAccess': False}
         thread = request.env[thread_model].with_context(active_test=False).search([('id', '=', thread_id)])
-        try:
-            thread.check_access_rights("write")
-            thread.check_access_rule("write")
-            res['hasWriteAccess'] = True
-        except AccessError:
-            pass
-        if 'activities' in request_list:
-            res['activities'] = thread.activity_ids.activity_format()
-        if 'attachments' in request_list:
-            res['attachments'] = thread.env['ir.attachment'].search([('res_id', '=', thread.id), ('res_model', '=', thread._name)], order='id desc')._attachment_format(commands=True)
-        if 'followers' in request_list:
-            res['followers'] = [{
-                'id': follower.id,
-                'partner_id': follower.partner_id.id,
-                'name': follower.name,
-                'display_name': follower.display_name,
-                'email': follower.email,
-                'is_active': follower.is_active,
-                'partner': follower.partner_id.mail_partner_format()[follower.partner_id],
-            } for follower in thread.message_follower_ids]
-        if 'suggestedRecipients' in request_list:
-            res['suggestedRecipients'] = thread._message_get_suggested_recipients()[thread.id]
-        return res
+        return thread._get_mail_thread_data(request_list)
 
     @http.route('/mail/thread/messages', methods=['POST'], type='json', auth='user')
     def mail_thread_messages(self, thread_model, thread_id, max_id=None, min_id=None, limit=30, **kwargs):
diff --git a/addons/mail/models/mail_thread.py b/addons/mail/models/mail_thread.py
index 69355b49598..a670c459195 100644
--- a/addons/mail/models/mail_thread.py
+++ b/addons/mail/models/mail_thread.py
@@ -25,7 +25,7 @@ from xmlrpc import client as xmlrpclib
 from markupsafe import Markup
 
 from odoo import _, api, exceptions, fields, models, tools, registry, SUPERUSER_ID, Command
-from odoo.exceptions import MissingError
+from odoo.exceptions import MissingError, AccessError
 from odoo.osv import expression
 from odoo.tools import is_html_empty
 from odoo.tools.misc import clean_context, split_every
@@ -3142,3 +3142,34 @@ class MailThread(models.AbstractModel):
         if 'company_id' in self:
             return self.company_id
         return False
+
+    def _get_mail_thread_data_attachments(self):
+        self.ensure_one()
+        return self.env['ir.attachment'].search([('res_id', '=', self.id), ('res_model', '=', self._name)], order='id desc')
+
+    def _get_mail_thread_data(self, request_list):
+        self.ensure_one()
+        res = {'hasWriteAccess': False}
+        try:
+            self.check_access_rights("write")
+            self.check_access_rule("write")
+            res['hasWriteAccess'] = True
+        except AccessError:
+            pass
+        if 'activities' in request_list:
+            res['activities'] = self.activity_ids.activity_format()
+        if 'attachments' in request_list:
+            res['attachments'] = self._get_mail_thread_data_attachments()._attachment_format(commands=True)
+        if 'followers' in request_list:
+            res['followers'] = [{
+                'id': follower.id,
+                'partner_id': follower.partner_id.id,
+                'name': follower.name,
+                'display_name': follower.display_name,
+                'email': follower.email,
+                'is_active': follower.is_active,
+                'partner': follower.partner_id.mail_partner_format()[follower.partner_id],
+            } for follower in self.message_follower_ids]
+        if 'suggestedRecipients' in request_list:
+            res['suggestedRecipients'] = self._message_get_suggested_recipients()[self.id]
+        return res