diff --git a/API.md b/API.md
index 05df63d9..4d52409d 100644
--- a/API.md
+++ b/API.md
@@ -1334,27 +1334,28 @@ One file together with its metadata posted to the site.
 
 ```json5
 {
-    "id":              <id>,
-    "creationTime":    <creation-time>,
-    "lastEditTime":    <last-edit-time>,
-    "safety":          <safety>,
-    "source":          <source>,
-    "type":            <type>,
-    "checksum":        <checksum>,
-    "canvasWidth":     <canvas-width>,
-    "canvasHeight":    <canvas-height>,
-    "contentUrl":      <content-url>,
-    "thumbnailUrl":    <thumbnail-url>,
-    "flags":           <flags>,
-    "tags":            <tags>,
-    "relations":       <relations>,
-    "notes":           <notes>,
-    "user":            <user>,
-    "score":           <score>,
-    "ownScore":        <own-score>,
-    "featureCount":    <feature-count>,
-    "lastFeatureTime": <last-feature-time>,
-    "favoritedBy":     <favorited-by>
+    "id":                 <id>,
+    "creationTime":       <creation-time>,
+    "lastEditTime":       <last-edit-time>,
+    "safety":             <safety>,
+    "source":             <source>,
+    "type":               <type>,
+    "checksum":           <checksum>,
+    "canvasWidth":        <canvas-width>,
+    "canvasHeight":       <canvas-height>,
+    "contentUrl":         <content-url>,
+    "thumbnailUrl":       <thumbnail-url>,
+    "flags":              <flags>,
+    "tags":               <tags>,
+    "relations":          <relations>,
+    "notes":              <notes>,
+    "user":               <user>,
+    "score":              <score>,
+    "ownScore":           <own-score>,
+    "featureCount":       <feature-count>,
+    "lastFeatureTime":    <last-feature-time>,
+    "favoritedBy":        <favorited-by>,
+    "hasCustomThumbnail": <has-custom-thumbnail>
 }
 ```
 
@@ -1403,6 +1404,7 @@ One file together with its metadata posted to the site.
 - `<last-feature-time>`: the last time the post was featured, formatted as per
   RFC 3339.
 - `<favorited-by>`: list of users, serialized as [user resources](#user).
+- `<has-custom-thumbnail>`: whether the post uses custom thumbnail.
 
 ## Detailed post
 **Description**
diff --git a/server/szurubooru/func/files.py b/server/szurubooru/func/files.py
index 2a0217b1..f5209327 100644
--- a/server/szurubooru/func/files.py
+++ b/server/szurubooru/func/files.py
@@ -9,6 +9,9 @@ def delete(path):
     if os.path.exists(full_path):
         os.unlink(full_path)
 
+def has(path):
+    return os.path.exists(_get_full_path(path))
+
 def get(path):
     full_path = _get_full_path(path)
     if not os.path.exists(full_path):
diff --git a/server/szurubooru/func/posts.py b/server/szurubooru/func/posts.py
index b79c117e..e12f30d7 100644
--- a/server/szurubooru/func/posts.py
+++ b/server/szurubooru/func/posts.py
@@ -85,6 +85,7 @@ def serialize_post(post, authenticated_user):
         'lastFeatureTime': post.last_feature_time,
         'favoritedBy': [users.serialize_user(rel.user, authenticated_user) \
             for rel in post.favorited_by],
+        'hasCustomThumbnail': files.has(get_post_thumbnail_backup_path(post)),
     }
 
     if authenticated_user:
diff --git a/server/szurubooru/tests/api/test_comment_creating.py b/server/szurubooru/tests/api/test_comment_creating.py
index 218239c0..ccb944d4 100644
--- a/server/szurubooru/tests/api/test_comment_creating.py
+++ b/server/szurubooru/tests/api/test_comment_creating.py
@@ -4,8 +4,10 @@ from szurubooru import api, db, errors
 from szurubooru.func import util, posts
 
 @pytest.fixture
-def test_ctx(config_injector, context_factory, post_factory, user_factory):
+def test_ctx(
+        tmpdir, config_injector, context_factory, post_factory, user_factory):
     config_injector({
+        'data_dir': str(tmpdir),
         'data_url': 'http://example.com',
         'ranks': ['anonymous', 'regular_user'],
         'rank_names': {'anonymous': 'Peasant', 'regular_user': 'Lord'},
diff --git a/server/szurubooru/tests/api/test_comment_rating.py b/server/szurubooru/tests/api/test_comment_rating.py
index 7d10700c..28c49086 100644
--- a/server/szurubooru/tests/api/test_comment_rating.py
+++ b/server/szurubooru/tests/api/test_comment_rating.py
@@ -4,8 +4,10 @@ from szurubooru import api, db, errors
 from szurubooru.func import util, comments, scores
 
 @pytest.fixture
-def test_ctx(config_injector, context_factory, user_factory, comment_factory):
+def test_ctx(
+        tmpdir, config_injector, context_factory, user_factory, comment_factory):
     config_injector({
+        'data_dir': str(tmpdir),
         'data_url': 'http://example.com',
         'ranks': ['anonymous', 'regular_user', 'mod'],
         'rank_names': {'anonymous': 'Peasant', 'regular_user': 'Lord'},
diff --git a/server/szurubooru/tests/api/test_comment_retrieving.py b/server/szurubooru/tests/api/test_comment_retrieving.py
index b0011b60..f0a2f746 100644
--- a/server/szurubooru/tests/api/test_comment_retrieving.py
+++ b/server/szurubooru/tests/api/test_comment_retrieving.py
@@ -4,8 +4,10 @@ from szurubooru import api, db, errors
 from szurubooru.func import util, comments
 
 @pytest.fixture
-def test_ctx(context_factory, config_injector, user_factory, comment_factory):
+def test_ctx(
+        tmpdir, context_factory, config_injector, user_factory, comment_factory):
     config_injector({
+        'data_dir': str(tmpdir),
         'data_url': 'http://example.com',
         'ranks': ['anonymous', 'regular_user', 'mod', 'admin'],
         'rank_names': {'regular_user': 'Peasant'},
diff --git a/server/szurubooru/tests/api/test_comment_updating.py b/server/szurubooru/tests/api/test_comment_updating.py
index 08ade5cc..16b2563d 100644
--- a/server/szurubooru/tests/api/test_comment_updating.py
+++ b/server/szurubooru/tests/api/test_comment_updating.py
@@ -4,8 +4,10 @@ from szurubooru import api, db, errors
 from szurubooru.func import util, comments
 
 @pytest.fixture
-def test_ctx(config_injector, context_factory, user_factory, comment_factory):
+def test_ctx(
+        tmpdir, config_injector, context_factory, user_factory, comment_factory):
     config_injector({
+        'data_dir': str(tmpdir),
         'data_url': 'http://example.com',
         'ranks': ['anonymous', 'regular_user', 'mod'],
         'rank_names': {'anonymous': 'Peasant', 'regular_user': 'Lord', 'mod': 'King'},
diff --git a/server/szurubooru/tests/api/test_post_favoriting.py b/server/szurubooru/tests/api/test_post_favoriting.py
index e5e627d2..d411bf02 100644
--- a/server/szurubooru/tests/api/test_post_favoriting.py
+++ b/server/szurubooru/tests/api/test_post_favoriting.py
@@ -4,8 +4,10 @@ from szurubooru import api, db, errors
 from szurubooru.func import util, posts
 
 @pytest.fixture
-def test_ctx(config_injector, context_factory, user_factory, post_factory):
+def test_ctx(
+        tmpdir, config_injector, context_factory, user_factory, post_factory):
     config_injector({
+        'data_dir': str(tmpdir),
         'data_url': 'http://example.com',
         'ranks': ['anonymous', 'regular_user', 'mod'],
         'rank_names': {'anonymous': 'Peasant', 'regular_user': 'Lord'},
diff --git a/server/szurubooru/tests/api/test_post_featuring.py b/server/szurubooru/tests/api/test_post_featuring.py
index 8a030f3d..cd8e6d66 100644
--- a/server/szurubooru/tests/api/test_post_featuring.py
+++ b/server/szurubooru/tests/api/test_post_featuring.py
@@ -4,8 +4,10 @@ from szurubooru import api, db, errors
 from szurubooru.func import util, posts
 
 @pytest.fixture
-def test_ctx(context_factory, config_injector, user_factory, post_factory):
+def test_ctx(
+        tmpdir, context_factory, config_injector, user_factory, post_factory):
     config_injector({
+        'data_dir': str(tmpdir),
         'data_url': 'http://example.com',
         'privileges': {
             'posts:feature': 'regular_user',
diff --git a/server/szurubooru/tests/api/test_post_rating.py b/server/szurubooru/tests/api/test_post_rating.py
index def80fa8..03873c13 100644
--- a/server/szurubooru/tests/api/test_post_rating.py
+++ b/server/szurubooru/tests/api/test_post_rating.py
@@ -3,8 +3,10 @@ from szurubooru import api, db, errors
 from szurubooru.func import util, posts, scores
 
 @pytest.fixture
-def test_ctx(config_injector, context_factory, user_factory, post_factory):
+def test_ctx(
+        tmpdir, config_injector, context_factory, user_factory, post_factory):
     config_injector({
+        'data_dir': str(tmpdir),
         'data_url': 'http://example.com',
         'ranks': ['anonymous', 'regular_user'],
         'rank_names': {'anonymous': 'Peasant', 'regular_user': 'Lord'},
diff --git a/server/szurubooru/tests/api/test_post_retrieving.py b/server/szurubooru/tests/api/test_post_retrieving.py
index ec39a9af..369fa790 100644
--- a/server/szurubooru/tests/api/test_post_retrieving.py
+++ b/server/szurubooru/tests/api/test_post_retrieving.py
@@ -4,8 +4,10 @@ from szurubooru import api, db, errors
 from szurubooru.func import util, posts
 
 @pytest.fixture
-def test_ctx(context_factory, config_injector, user_factory, post_factory):
+def test_ctx(
+        tmpdir, context_factory, config_injector, user_factory, post_factory):
     config_injector({
+        'data_dir': str(tmpdir),
         'data_url': 'http://example.com',
         'privileges': {
             'posts:list': 'regular_user',
diff --git a/server/szurubooru/tests/func/test_posts.py b/server/szurubooru/tests/func/test_posts.py
index c0c458c1..f24c1f6c 100644
--- a/server/szurubooru/tests/func/test_posts.py
+++ b/server/szurubooru/tests/func/test_posts.py
@@ -65,7 +65,8 @@ def test_serialize_empty_post():
     assert posts.serialize_post(None, None) is None
 
 def test_serialize_post(post_factory, user_factory, tag_factory):
-    with unittest.mock.patch('szurubooru.func.users.serialize_user'):
+    with unittest.mock.patch('szurubooru.func.users.serialize_user'), \
+        unittest.mock.patch('szurubooru.func.posts.files.has', return_value=True):
         users.serialize_user.side_effect = lambda user, auth_user: user.name
 
         auth_user = user_factory(name='auth user')
@@ -140,6 +141,7 @@ def test_serialize_post(post_factory, user_factory, tag_factory):
         'featureCount': 1,
         'lastFeatureTime': datetime.datetime(1999, 1, 1),
         'favoritedBy': ['fav1'],
+        'hasCustomThumbnail': True,
     }
 
 def test_serialize_post_with_details(post_factory, comment_factory, user_factory):