diff --git a/benedict/dicts/io/io_dict.py b/benedict/dicts/io/io_dict.py
index 1223e3c..dd13d24 100644
--- a/benedict/dicts/io/io_dict.py
+++ b/benedict/dicts/io/io_dict.py
@@ -20,7 +20,8 @@ class IODict(dict):
 
     @staticmethod
     def _decode_init(s, **kwargs):
-        default_format = io_util.autodetect_format(s, default='json')
+        autodetected_format = io_util.autodetect_format(s)
+        default_format = autodetected_format or 'json'
         format = kwargs.pop('format', default_format).lower()
         if format in ['b64', 'base64']:
             kwargs.setdefault('subformat', 'json')
diff --git a/benedict/dicts/io/io_util.py b/benedict/dicts/io/io_util.py
index e936620..9117699 100644
--- a/benedict/dicts/io/io_util.py
+++ b/benedict/dicts/io/io_util.py
@@ -8,12 +8,10 @@ import os
 import requests
 
 
-def autodetect_format(s, default=None):
-    if is_data(s):
-        return default
-    elif is_url(s) or is_filepath(s):
+def autodetect_format(s):
+    if is_url(s) or is_filepath(s):
         return get_format_by_path(s)
-    return default
+    return None
 
 
 def decode(s, format, **kwargs):
@@ -38,8 +36,9 @@ def is_data(s):
 
 
 def is_filepath(s):
-    return any([s.endswith(extension)
-                for extension in get_serializers_extensions()])
+    if any([s.endswith(ext) for ext in get_serializers_extensions()]):
+        return True
+    return os.path.isfile(s)
 
 
 def is_url(s):
diff --git a/tests/dicts/io/input/valid-content.json.txt b/tests/dicts/io/input/valid-content.json.txt
new file mode 100644
index 0000000..74a974d
--- /dev/null
+++ b/tests/dicts/io/input/valid-content.json.txt
@@ -0,0 +1 @@
+{"a": 1, "b": 2, "c": 3, "x": 7, "y": 8, "z": 9}
\ No newline at end of file
diff --git a/tests/dicts/io/test_io_dict_json.py b/tests/dicts/io/test_io_dict_json.py
index 43bb30c..57adaa3 100644
--- a/tests/dicts/io/test_io_dict_json.py
+++ b/tests/dicts/io/test_io_dict_json.py
@@ -82,6 +82,18 @@ class io_dict_json_test_case(io_dict_test_case):
         d = IODict(filepath)
         self.assertTrue(isinstance(d, dict))
 
+    def test_from_json_with_valid_file_valid_content_but_unexpected_extension(self):
+        filepath = self.input_path('valid-content.json.txt')
+        # static method
+        d = IODict.from_json(filepath)
+        self.assertTrue(isinstance(d, dict))
+        # constructor
+        d = IODict(filepath, format='json')
+        self.assertTrue(isinstance(d, dict))
+        # constructor with format autodetection
+        d = IODict(filepath)
+        self.assertTrue(isinstance(d, dict))
+
     def test_from_json_with_valid_file_valid_content_invalid_format(self):
         filepath = self.input_path('valid-content.base64')
         with self.assertRaises(ValueError):
diff --git a/tests/dicts/io/test_io_util.py b/tests/dicts/io/test_io_util.py
index 71e929c..e3dd61f 100644
--- a/tests/dicts/io/test_io_util.py
+++ b/tests/dicts/io/test_io_util.py
@@ -15,10 +15,6 @@ class io_util_test_case(unittest.TestCase):
         s = '{"a": 1, "b": 2, "c": 3}'
         self.assertEqual(io_util.autodetect_format(s), None)
 
-    def test_autodetect_format_by_data_with_default(self):
-        s = '{"a": 1, "b": 2, "c": 3}'
-        self.assertEqual(io_util.autodetect_format(s, default='json'), 'json')
-
     def test_autodetect_format_by_path(self):
         s = 'path-to/data.xml'
         self.assertEqual(io_util.autodetect_format(s), 'xml')