diff --git a/addons/mail/static/src/components/attachment/attachment.js b/addons/mail/static/src/components/attachment/attachment.js
index 76922ed7a3c..a17ed341b9a 100644
--- a/addons/mail/static/src/components/attachment/attachment.js
+++ b/addons/mail/static/src/components/attachment/attachment.js
@@ -71,6 +71,12 @@ class Attachment extends Component {
         if (this.attachment.fileType !== 'image') {
             return '';
         }
+        if (this.env.isQUnitTest) {
+            // background-image:url is hardly mockable, and attachments in
+            // QUnit tests do not actually exist in DB, so style should not
+            // be fetched at all.
+            return '';
+        }
         let size;
         if (this.detailsMode === 'card') {
             size = '38x38';
diff --git a/addons/mail/static/src/env/test_env.js b/addons/mail/static/src/env/test_env.js
index fb16caaaf5e..048bfc8d65f 100644
--- a/addons/mail/static/src/env/test_env.js
+++ b/addons/mail/static/src/env/test_env.js
@@ -56,6 +56,13 @@ function addMessagingToEnv(providedEnv = {}) {
             }
             return this.messaging.isInitialized;
         },
+        /**
+         * States whether the environment is in QUnit test or not.
+         *
+         * Useful to prevent some behaviour in QUnit tests, like applying
+         * style of attachment that uses url.
+         */
+        isQUnitTest: true,
         loadingBaseDelayDuration: providedEnv.loadingBaseDelayDuration || 0,
         messaging: undefined,
         messagingCreatedPromise: makeDeferred(),
diff --git a/addons/mail/static/src/js/main.js b/addons/mail/static/src/js/main.js
index f8199c4dc34..d19a8edb8e9 100644
--- a/addons/mail/static/src/js/main.js
+++ b/addons/mail/static/src/js/main.js
@@ -71,6 +71,13 @@ Object.assign(env, {
         }
         return this.messaging.isInitialized;
     },
+    /**
+     * States whether the environment is in QUnit test or not.
+     *
+     * Useful to prevent some behaviour in QUnit tests, like applying
+     * style of attachment that uses url.
+     */
+    isQUnitTest: false,
     loadingBaseDelayDuration: 400,
     messaging: undefined,
     messagingBus: new EventBus(),