diff --git a/API.md b/API.md
index f4a1f61a..79c67ab3 100644
--- a/API.md
+++ b/API.md
@@ -1440,7 +1440,7 @@ experience.
     "name":  <name>,
     "color": <color>,
     "usages": <usages>
-    "default": <is_default>
+    "default": <is-default>
 }
 ```
 
@@ -1449,7 +1449,7 @@ experience.
 - `<name>`: the category name.
 - `<color>`: the category color.
 - `<usages>`: how many tags is the given category used with.
-- `<is_default>`: whether the tag category is the default one.
+- `<is-default>`: whether the tag category is the default one.
 
 ## Detailed tag category
 **Description**
@@ -1560,7 +1560,8 @@ One file together with its metadata posted to the site.
     "featureCount":       <feature-count>,
     "lastFeatureTime":    <last-feature-time>,
     "favoritedBy":        <favorited-by>,
-    "hasCustomThumbnail": <has-custom-thumbnail>
+    "hasCustomThumbnail": <has-custom-thumbnail>,
+    "mimeType":           <mime-type>
 }
 ```
 
@@ -1610,6 +1611,8 @@ One file together with its metadata posted to the site.
   RFC 3339.
 - `<favorited-by>`: list of users, serialized as [user resources](#user).
 - `<has-custom-thumbnail>`: whether the post uses custom thumbnail.
+- `<mime-type>`: subsidiary to `<type>`, used to tell exact content format;
+  useful for `<video>` tags for instance.
 
 ## Detailed post
 **Description**
diff --git a/server/szurubooru/func/posts.py b/server/szurubooru/func/posts.py
index 7dce0e9c..f7015f4f 100644
--- a/server/szurubooru/func/posts.py
+++ b/server/szurubooru/func/posts.py
@@ -89,6 +89,7 @@ def serialize_post(post, authenticated_user):
         'favoritedBy': [users.serialize_user(rel.user, authenticated_user) \
             for rel in post.favorited_by],
         'hasCustomThumbnail': files.has(get_post_thumbnail_backup_path(post)),
+        'mimeType': post.mime_type,
     }
 
     if authenticated_user:
diff --git a/server/szurubooru/tests/func/test_posts.py b/server/szurubooru/tests/func/test_posts.py
index f24c1f6c..e032586f 100644
--- a/server/szurubooru/tests/func/test_posts.py
+++ b/server/szurubooru/tests/func/test_posts.py
@@ -64,7 +64,8 @@ def test_serialize_note():
 def test_serialize_empty_post():
     assert posts.serialize_post(None, None) is None
 
-def test_serialize_post(post_factory, user_factory, tag_factory):
+def test_serialize_post(post_factory, user_factory, tag_factory, config_injector):
+    config_injector({'data_url': 'http://example.com/'})
     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
@@ -142,6 +143,7 @@ def test_serialize_post(post_factory, user_factory, tag_factory):
         'lastFeatureTime': datetime.datetime(1999, 1, 1),
         'favoritedBy': ['fav1'],
         'hasCustomThumbnail': True,
+        'mimeType': 'image/jpeg',
     }
 
 def test_serialize_post_with_details(post_factory, comment_factory, user_factory):