diff --git a/pytest.ini b/pytest.ini
new file mode 100644
index 0000000..de19c9f
--- /dev/null
+++ b/pytest.ini
@@ -0,0 +1,2 @@
+[pytest]
+testpaths = tests
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diff --git a/tests/test_validation.py b/tests/test_validation.py
new file mode 100644
index 0000000..836a235
--- /dev/null
+++ b/tests/test_validation.py
@@ -0,0 +1,24 @@
+import sys
+import os
+import pytest
+
+# Add root directory to Python path
+sys.path.append(os.path.abspath(os.path.join(os.path.dirname(__file__), '..')))
+
+from utils.validation import validate_file
+
+
+def test_valid_file(tmp_path):
+    file = tmp_path / "test.nii"
+    file.write_text("dummy")
+    assert validate_file(str(file)) is True
+
+
+def test_invalid_extension():
+    with pytest.raises(ValueError):
+        validate_file("file.txt")
+
+
+def test_missing_file():
+    with pytest.raises(FileNotFoundError):
+        validate_file("missing.nii")
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diff --git a/utils/__init__.py b/utils/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/utils/validation.py b/utils/validation.py
new file mode 100644
index 0000000..f538fae
--- /dev/null
+++ b/utils/validation.py
@@ -0,0 +1,13 @@
+import os
+
+def validate_file(path: str) -> bool:
+
+    # FIRST check extension
+    if not path.endswith(".nii"):
+        raise ValueError("Only .nii files allowed")
+
+    # THEN check existence
+    if not os.path.exists(path):
+        raise FileNotFoundError(f"{path} not found")
+
+    return True
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