Give FileFormat XML for XML files on every platform

mathics/eval/import_export/importexport.py

@@ -175,9 +175,20 @@ def eval_ImageExport(expr, path: Optional[str] = None) -> Expression:
     return Expression(SymbolByteArray, ByteArray(expr_pil.tobytes()))
 
 
+# MIME types for which mimetypes.guess_extension() depends on the
+# platform. For example, without an /etc/mime.types file (as on macOS),
+# Python gives ".xsl" for "application/xml".
+MIMETYPE_TO_SHORTNAME_OVERRIDE: Final[Dict[str, str]] = {
+    "application/xml": "XML",
+    "text/xml": "XML",
+}
+
+
 def filetype_from_mime_content(mime_content_name: str) -> str:
 
-    if mime_content_name in MIMETYPE_TO_SHORTNAME:
+    if mime_content_name in MIMETYPE_TO_SHORTNAME_OVERRIDE:
+        short_name = MIMETYPE_TO_SHORTNAME_OVERRIDE[mime_content_name]
+    elif mime_content_name in MIMETYPE_TO_SHORTNAME:
         short_name = MIMETYPE_TO_SHORTNAME[mime_content_name]
     else:
         # Map MIME type to a standard extension using the stdlib

test/builtin/import_export/test_importexport.py

@@ -401,3 +401,21 @@ def test_inividually():
 def test_b64encode(str_expr, msgs, str_expected, fail_msg):
     """special case"""
     check_evaluation_as_in_cli(str_expr, str_expected, fail_msg, msgs)
+
+
+@pytest.mark.parametrize(
+    ("mime_type", "short_name"),
+    [
+        ("application/xml", "XML"),
+        ("text/xml", "XML"),
+        ("image/jpeg", "JPG"),
+        ("image/png", "PNG"),
+    ],
+)
+def test_filetype_from_mime_content(mime_type, short_name):
+    """The short name of a MIME type must not depend on the platform's
+    mimetypes database (without /etc/mime.types, Python maps
+    "application/xml" to ".xsl")."""
+    from mathics.eval.import_export.importexport import filetype_from_mime_content
+
+    assert filetype_from_mime_content(mime_type).lstrip(".") == short_name