diff --git a/mdto/helpers.py b/mdto/helpers.py
new file mode 100644
index 0000000..55bed3f
--- /dev/null
+++ b/mdto/helpers.py
@@ -0,0 +1,30 @@
+# Private helper methods
+from typing import TextIO
+from pathlib import Path
+
+
+def process_file(file_or_filename) -> TextIO:
+    """Return file-object if input is already a file.
+    Otherwise, assume the argument is a path, and convert
+    it to a new file-object.
+
+    Note:
+        The returned file-object is always in read-only mode.
+    """
+
+    # filename or path?
+    if isinstance(file_or_filename, (str, Path)):
+        return open(file_or_filename, "r")
+    # file-like object?
+    elif hasattr(file_or_filename, "read"):
+        # if file-like object, force it to be opened read-only
+        if file_or_filename.writable():
+            filename = file_or_filename.name
+            file_or_filename.close()  # FIXME: callers might get confused by suddenly closed files
+            return open(filename, "r")
+        else:
+            return file_or_filename
+    else:
+        raise TypeError(
+            f"Expected file object or str, but got value of type {type(file_or_filename)}"
+        )