Back-fill Description-Content-Type according to readme suffix

According to PEP 621, the backend should fill-in the content-type if the
`readme` field is passed as a string. The value is derived from the
extension of the file (an error should be raised when that is not
possible).

Previously all READMEs were wrongly assumed rst.
This error was reported in:

https://discuss.python.org/t/help-testing-experimental-features-in-setuptools/13821/4

setuptools/config/_apply_pyprojecttoml.py

@@ -74,18 +74,39 @@ def _set_config(dist: "Distribution", field: str, value: Any):
         setattr(dist, field, value)
 
 
+_CONTENT_TYPES = {
+    ".md": "text/markdown",
+    ".rst": "text/x-rst",
+    ".txt": "text/plain",
+}
+
+
+def _guess_content_type(file: str) -> Optional[str]:
+    _, ext = os.path.splitext(file.lower())
+    if not ext:
+        return None
+
+    if ext in _CONTENT_TYPES:
+        return _CONTENT_TYPES[ext]
+
+    valid = ", ".join(f"{k} ({v})" for k, v in _CONTENT_TYPES.items())
+    msg = f"only the following file extensions are recognized: {valid}."
+    raise ValueError(f"Undefined content type for {file}, {msg}")
+
+
 def _long_description(dist: "Distribution", val: _DictOrStr, root_dir: _Path):
     from setuptools.config import expand
 
     if isinstance(val, str):
         text = expand.read_files(val, root_dir)
-        ctype = "text/x-rst"
+        ctype = _guess_content_type(val)
     else:
         text = val.get("text") or expand.read_files(val.get("file", []), root_dir)
         ctype = val["content-type"]
 
     _set_config(dist, "long_description", text)
-    _set_config(dist, "long_description_content_type", ctype)
+    if ctype:
+        _set_config(dist, "long_description_content_type", ctype)
 
 
 def _license(dist: "Distribution", val: Union[str, dict], _root_dir):

setuptools/tests/config/test_apply_pyprojecttoml.py

@@ -129,18 +129,53 @@ def main_tomatoes(): pass
 """
 
 
-def test_pep621_example(tmp_path):
-    """Make sure the example in PEP 621 works"""
+def _pep621_example_project(tmp_path, readme="README.rst"):
     pyproject = tmp_path / "pyproject.toml"
-    pyproject.write_text(PEP621_EXAMPLE)
+    text = PEP621_EXAMPLE
+    replacements = {'readme = "README.rst"': f'readme = "{readme}"'}
+    for orig, subst in replacements.items():
+        text = text.replace(orig, subst)
+    pyproject.write_text(text)
+
     (tmp_path / "README.rst").write_text("hello world")
     (tmp_path / "LICENSE.txt").write_text("--- LICENSE stub ---")
     (tmp_path / "spam.py").write_text(PEP621_EXAMPLE_SCRIPT)
+    return pyproject
+
 
+def test_pep621_example(tmp_path):
+    """Make sure the example in PEP 621 works"""
+    pyproject = _pep621_example_project(tmp_path)
     dist = pyprojecttoml.apply_configuration(Distribution(), pyproject)
     assert set(dist.metadata.license_files) == {"LICENSE.txt"}
 
 
+@pytest.mark.parametrize(
+    "readme, ctype",
+    [
+        ("Readme.txt", "text/plain"),
+        ("readme.md", "text/markdown"),
+        ("text.rst", "text/x-rst"),
+    ]
+)
+def test_readme_content_type(tmp_path, readme, ctype):
+    pyproject = _pep621_example_project(tmp_path, readme)
+    dist = pyprojecttoml.apply_configuration(Distribution(), pyproject)
+    assert dist.metadata.long_description_content_type == ctype
+
+
+def test_undefined_content_type(tmp_path):
+    pyproject = _pep621_example_project(tmp_path, "README.tex")
+    with pytest.raises(ValueError, match="Undefined content type for README.tex"):
+        pyprojecttoml.apply_configuration(Distribution(), pyproject)
+
+
+def test_no_explicit_content_type_for_missing_extension(tmp_path):
+    pyproject = _pep621_example_project(tmp_path, "README")
+    dist = pyprojecttoml.apply_configuration(Distribution(), pyproject)
+    assert dist.metadata.long_description_content_type is None
+
+
 # --- Auxiliary Functions ---