raise clear error when plain file references a binary resource

Referencing a binary file from a .plain file crashed with
"cannot access local variable 'content_text' where it is not
associated with a value" because open_from() swallowed
UnicodeDecodeError but still returned the never-assigned variable.
Now the decode error propagates and load_linked_resources() converts
it into a PlainSyntaxError naming the offending file.

Author: pedjaradenkovic

file_utils.py

@@ -8,6 +8,7 @@
 
 import plain_spec
 from plain2code_console import console
+from plain2code_exceptions import PlainSyntaxError
 from plain2code_nodes import Plain2CodeIncludeTag, Plain2CodeLoaderMixin
 from plain_modules import CODEPLAIN_MEMORY_SUBFOLDER, CODEPLAIN_METADATA_FOLDER
 
@@ -217,13 +218,7 @@ def open_from(dirs, file_name):
 
         with open(full_file_name, "rb") as f:
             content = f.read()
-            try:
-                content_text = content.decode("utf-8")
-            except UnicodeDecodeError:
-                console.debug(
-                    f"WARNING! Error loading {file_name} ({file_name}). File is not a text file. Skipping it."
-                )
-            return content_text
+        return content.decode("utf-8")
 
     return None
 
@@ -236,7 +231,13 @@ def load_linked_resources(template_dirs: list[str], resources_list):
         if file_name in linked_resources:
             continue
 
-        content = open_from(template_dirs, file_name)
+        try:
+            content = open_from(template_dirs, file_name)
+        except UnicodeDecodeError:
+            raise PlainSyntaxError(
+                f"Plain syntax error: Referenced resource '{file_name}' is a binary file. "
+                f"Only text files (e.g. .md, .txt, .json, .yaml) can be referenced from a .plain file."
+            )
 
         if content is None:
             raise FileNotFoundError(f"""File not found:

tests/test_file_utils.py

@@ -0,0 +1,40 @@
+import os
+import tempfile
+
+import pytest
+
+from file_utils import load_linked_resources
+from plain2code_exceptions import PlainSyntaxError
+
+
+@pytest.fixture
+def template_dir():
+    with tempfile.TemporaryDirectory() as d:
+        yield d
+
+
+def test_load_linked_resources_text_file(template_dir):
+    file_path = os.path.join(template_dir, "notes.md")
+    with open(file_path, "w") as f:
+        f.write("# hello")
+
+    result = load_linked_resources([template_dir], [{"text": "Notes", "target": "notes.md"}])
+
+    assert result == {"notes.md": "# hello"}
+
+
+def test_load_linked_resources_binary_file_raises_plain_syntax_error(template_dir):
+    file_path = os.path.join(template_dir, "icon.png")
+    with open(file_path, "wb") as f:
+        f.write(b"\x89PNG\r\n\x1a\n\x00\x00\x00\rIHDR\xff\xfe\xfd")
+
+    with pytest.raises(PlainSyntaxError) as exc_info:
+        load_linked_resources([template_dir], [{"text": "Icon", "target": "icon.png"}])
+
+    assert "icon.png" in str(exc_info.value)
+    assert "binary file" in str(exc_info.value)
+
+
+def test_load_linked_resources_missing_file_raises_file_not_found(template_dir):
+    with pytest.raises(FileNotFoundError):
+        load_linked_resources([template_dir], [{"text": "Missing", "target": "missing.md"}])