diff --git a/masque/file/gdsii.py b/masque/file/gdsii.py
index 3d97fe3..68b69a9 100644
--- a/masque/file/gdsii.py
+++ b/masque/file/gdsii.py
@@ -35,6 +35,7 @@ import gdsii.structure
 import gdsii.elements
 
 from .utils import mangle_name, make_dose_table, dose2dtype, dtype2dose, clean_pattern_vertices
+from .utils import is_gzipped
 from .. import Pattern, SubPattern, PatternError, Label, Shape
 from ..shapes import Polygon, Path
 from ..repetition import Grid
@@ -190,7 +191,7 @@ def readfile(filename: Union[str, pathlib.Path],
     """
     Wrapper for `masque.file.gdsii.read()` that takes a filename or path instead of a stream.
 
-    Will automatically decompress files with a .gz suffix.
+    Will automatically decompress gzipped files.
 
     Args:
         filename: Filename to save to.
@@ -198,7 +199,7 @@ def readfile(filename: Union[str, pathlib.Path],
         **kwargs: passed to `masque.file.gdsii.read`
     """
     path = pathlib.Path(filename)
-    if path.suffix == '.gz':
+    if is_gzipped(path):
         open_func: Callable = gzip.open
     else:
         open_func = open
diff --git a/masque/file/klamath.py b/masque/file/klamath.py
index 192ce9f..ba89638 100644
--- a/masque/file/klamath.py
+++ b/masque/file/klamath.py
@@ -34,7 +34,7 @@ import numpy        # type: ignore
 import klamath
 from klamath import records
 
-from .utils import mangle_name, make_dose_table, dose2dtype, dtype2dose
+from .utils import mangle_name, make_dose_table, dose2dtype, dtype2dose, is_gzipped
 from .. import Pattern, SubPattern, PatternError, Label, Shape
 from ..shapes import Polygon, Path
 from ..repetition import Grid
@@ -170,7 +170,7 @@ def readfile(filename: Union[str, pathlib.Path],
     """
     Wrapper for `masque.file.gdsii.read()` that takes a filename or path instead of a stream.
 
-    Will automatically decompress files with a .gz suffix.
+    Will automatically decompress gzipped files.
 
     Args:
         filename: Filename to save to.
@@ -178,7 +178,7 @@ def readfile(filename: Union[str, pathlib.Path],
         **kwargs: passed to `masque.file.gdsii.read`
     """
     path = pathlib.Path(filename)
-    if path.suffix == '.gz':
+    if is_gzipped(path):
         open_func: Callable = gzip.open
     else:
         open_func = open
diff --git a/masque/file/oasis.py b/masque/file/oasis.py
index 8d9e074..5e20121 100644
--- a/masque/file/oasis.py
+++ b/masque/file/oasis.py
@@ -27,7 +27,7 @@ import fatamorgana
 import fatamorgana.records as fatrec
 from fatamorgana.basic import PathExtensionScheme, AString, NString, PropStringReference
 
-from .utils import mangle_name, make_dose_table, clean_pattern_vertices
+from .utils import mangle_name, make_dose_table, clean_pattern_vertices, is_gzipped
 from .. import Pattern, SubPattern, PatternError, Label, Shape
 from ..shapes import Polygon, Path, Circle
 from ..repetition import Grid, Arbitrary, Repetition
@@ -207,7 +207,7 @@ def readfile(filename: Union[str, pathlib.Path],
     """
     Wrapper for `oasis.read()` that takes a filename or path instead of a stream.
 
-    Will automatically decompress files with a .gz suffix.
+    Will automatically decompress gzipped files.
 
     Args:
         filename: Filename to save to.
@@ -215,7 +215,7 @@ def readfile(filename: Union[str, pathlib.Path],
         **kwargs: passed to `oasis.read`
     """
     path = pathlib.Path(filename)
-    if path.suffix == '.gz':
+    if is_gzipped(path):
         open_func: Callable = gzip.open
     else:
         open_func = open
diff --git a/masque/file/utils.py b/masque/file/utils.py
index 30b2808..6239a1a 100644
--- a/masque/file/utils.py
+++ b/masque/file/utils.py
@@ -4,6 +4,8 @@ Helper functions for file reading and writing
 from typing import Set, Tuple, List
 import re
 import copy
+import gzip
+import pathlib
 
 from .. import Pattern, PatternError
 from ..shapes import Polygon, Path
@@ -176,3 +178,9 @@ def dose2dtype(patterns: List[Pattern],
             subpat.pattern = new_pats[(id(subpat.pattern), dose_mult)]
 
     return patterns, dose_vals_list
+
+
+def is_gzipped(path: pathlib.Path) -> bool:
+    with open(path, 'rb') as stream:
+        magic_bytes = stream.read(2)
+        return magic_bytes == b'\x1f\x8b'