diff --git a/notebook/services/contents/filemanager.py b/notebook/services/contents/filemanager.py
index ff8ac7d96..637ff52af 100644
--- a/notebook/services/contents/filemanager.py
+++ b/notebook/services/contents/filemanager.py
@@ -278,18 +278,20 @@ class FileContentsManager(FileManagerMixin, ContentsManager):
         model['type'] = 'file'
 
         os_path = self._get_os_path(path)
+        model['mimetype'] = mimetypes.guess_type(os_path)[0]
 
         if content:
             content, format = self._read_file(os_path, format)
-            default_mime = {
-                'text': 'text/plain',
-                'base64': 'application/octet-stream'
-            }[format]
+            if model['mimetype'] is None:
+                default_mime = {
+                    'text': 'text/plain',
+                    'base64': 'application/octet-stream'
+                }[format]
+                model['mimetype'] = default_mime
 
             model.update(
                 content=content,
                 format=format,
-                mimetype=mimetypes.guess_type(os_path)[0] or default_mime,
             )
 
         return model
diff --git a/notebook/services/contents/handlers.py b/notebook/services/contents/handlers.py
index 2a0a4cca5..521aae5fd 100644
--- a/notebook/services/contents/handlers.py
+++ b/notebook/services/contents/handlers.py
@@ -55,9 +55,6 @@ def validate_model(model, expect_content):
         )
 
     maybe_none_keys = ['content', 'format']
-    if model['type'] == 'file':
-        # mimetype should be populated only for file models
-        maybe_none_keys.append('mimetype')
     if expect_content:
         errors = [key for key in maybe_none_keys if model[key] is None]
         if errors: