diff --git a/src/tests/test_imghdr_stub.py b/src/tests/test_imghdr_stub.py
new file mode 100644
index 00000000..bcc119ac
--- /dev/null
+++ b/src/tests/test_imghdr_stub.py
@@ -0,0 +1,191 @@
+import pytest
+import io
+from utils.imghdr_stub import what
+
+
+def test_what_with_bytes_io_jpeg():
+    """Test what() with a BytesIO object containing JPEG header."""
+    # JPEG magic number: FF D8 FF E0 .. .. JFIF
+    data = b"\xff\xd8\xff\xe0\x00\x10JFIF" + b"\x00" * 20
+    f = io.BytesIO(data)
+    assert what(f) == "jpeg"
+
+
+def test_what_with_bytes_io_png():
+    """Test what() with a BytesIO object containing PNG header."""
+    # PNG magic number: 89 50 4E 47 0D 0A 1A 0A
+    data = b"\x89PNG\r\n\x1a\n" + b"\x00" * 24
+    f = io.BytesIO(data)
+    assert what(f) == "png"
+
+
+def test_what_with_bytes_io_gif87a():
+    """Test what() with a BytesIO object containing GIF87a header."""
+    data = b"GIF87a" + b"\x00" * 26
+    f = io.BytesIO(data)
+    assert what(f) == "gif"
+
+
+def test_what_with_bytes_io_gif89a():
+    """Test what() with a BytesIO object containing GIF89a header."""
+    data = b"GIF89a" + b"\x00" * 26
+    f = io.BytesIO(data)
+    assert what(f) == "gif"
+
+
+def test_what_with_bytes_io_tiff_intel():
+    """Test what() with a BytesIO object containing TIFF (Intel) header."""
+    data = b"II" + b"\x00" * 30
+    f = io.BytesIO(data)
+    assert what(f) == "tiff"
+
+
+def test_what_with_bytes_io_tiff_motorola():
+    """Test what() with a BytesIO object containing TIFF (Motorola) header."""
+    data = b"MM" + b"\x00" * 30
+    f = io.BytesIO(data)
+    assert what(f) == "tiff"
+
+
+def test_what_with_bytes_io_rgb():
+    """Test what() with a BytesIO object containing SGI RGB header."""
+    data = b"\x01\xda" + b"\x00" * 30
+    f = io.BytesIO(data)
+    assert what(f) == "rgb"
+
+
+def test_what_with_bytes_io_pbm():
+    """Test what() with a BytesIO object containing PBM header."""
+    # P1, P4 followed by whitespace
+    data = b"P1 " + b"\x00" * 29
+    f = io.BytesIO(data)
+    assert what(f) == "pbm"
+
+    data = b"P4\n" + b"\x00" * 29
+    f = io.BytesIO(data)
+    assert what(f) == "pbm"
+
+
+def test_what_with_bytes_io_pgm():
+    """Test what() with a BytesIO object containing PGM header."""
+    # P2, P5 followed by whitespace
+    data = b"P2\t" + b"\x00" * 29
+    f = io.BytesIO(data)
+    assert what(f) == "pgm"
+
+    data = b"P5\r" + b"\x00" * 29
+    f = io.BytesIO(data)
+    assert what(f) == "pgm"
+
+
+def test_what_with_bytes_io_ppm():
+    """Test what() with a BytesIO object containing PPM header."""
+    # P3, P6 followed by whitespace
+    data = b"P3\n" + b"\x00" * 29
+    f = io.BytesIO(data)
+    assert what(f) == "ppm"
+
+    data = b"P6 " + b"\x00" * 29
+    f = io.BytesIO(data)
+    assert what(f) == "ppm"
+
+
+def test_what_with_bytes_io_rast():
+    """Test what() with a BytesIO object containing Sun Raster header."""
+    data = b"\x59\xa6\x6a\x95" + b"\x00" * 28
+    f = io.BytesIO(data)
+    assert what(f) == "rast"
+
+
+def test_what_with_bytes_io_xbm():
+    """Test what() with a BytesIO object containing XBM header."""
+    data = b"#define " + b"\x00" * 24
+    f = io.BytesIO(data)
+    assert what(f) == "xbm"
+
+
+def test_what_with_bytes_io_bmp():
+    """Test what() with a BytesIO object containing BMP header."""
+    data = b"BM" + b"\x00" * 30
+    f = io.BytesIO(data)
+    assert what(f) == "bmp"
+
+
+def test_what_with_bytes_io_webp():
+    """Test what() with a BytesIO object containing WebP header."""
+    # RIFF .... WEBP
+    data = b"RIFF" + b"\x00\x00\x00\x00" + b"WEBP" + b"\x00" * 20
+    f = io.BytesIO(data)
+    assert what(f) == "webp"
+
+
+def test_what_with_bytes_io_exr():
+    """Test what() with a BytesIO object containing OpenEXR header."""
+    data = b"\x76\x2f\x31\x01" + b"\x00" * 28
+    f = io.BytesIO(data)
+    assert what(f) == "exr"
+
+
+def test_what_returns_none_for_unknown():
+    """Test what() returns None for unknown file types."""
+    f = io.BytesIO(b"\x00" * 32)
+    assert what(f) is None
+
+
+def test_what_with_file_path_str(tmp_path):
+    """Test what() with a file path string."""
+    p = tmp_path / "test_image.png"
+    p.write_bytes(b"\x89PNG\r\n\x1a\n" + b"\x00" * 24)
+    assert what(str(p)) == "png"
+
+
+def test_what_with_path_object(tmp_path):
+    """Test what() with a pathlib.Path object."""
+    p = tmp_path / "test_image.jpg"
+    p.write_bytes(b"\xff\xd8\xff\xe0\x00\x10JFIF" + b"\x00" * 20)
+    assert what(p) == "jpeg"
+
+
+def test_what_with_explicit_header():
+    """Test what() with explicit header provided."""
+    h = b"\x89PNG\r\n\x1a\n" + b"\x00" * 24
+    assert what(None, h) == "png"
+
+
+def test_what_preserves_file_position():
+    """Test that what() preserves the file position when passed a file object."""
+    data = b"prefix" + b"\x89PNG\r\n\x1a\n" + b"\x00" * 24
+    f = io.BytesIO(data)
+    f.seek(6)  # Skip prefix
+    original_pos = f.tell()
+
+    res = what(f)
+
+    assert res == "png"
+    assert f.tell() == original_pos
+
+def test_what_with_bytes_io_jpeg_exif():
+    """Test what() with a BytesIO object containing JPEG Exif header."""
+    # JPEG magic number: FF D8 FF E0 .. .. Exif
+    data = b"\xff\xd8\xff\xe0\x00\x10Exif" + b"\x00" * 20
+    f = io.BytesIO(data)
+    assert what(f) == "jpeg"
+
+
+def test_what_with_empty_file(tmp_path):
+    """Test what() with an empty file."""
+    p = tmp_path / "empty"
+    p.touch()
+    assert what(str(p)) is None
+
+
+def test_what_with_empty_bytes_io():
+    """Test what() with an empty BytesIO."""
+    f = io.BytesIO(b"")
+    assert what(f) is None
+
+
+def test_what_with_short_file_bmp():
+    """Test what() with a short file containing just the magic number."""
+    f = io.BytesIO(b"BM")
+    assert what(f) == "bmp"