xlsx: warn when a .xlsx file is not a ZIP package and is read as text (#3975)

crates/engine/src/io.rs

@@ -350,7 +350,13 @@ fn read_plain(name: &str, bytes: &[u8], csv_as_text: bool) -> Result<(Workbook,
             let opts = gridcraft_xlsx::CsvOptions { delimiter: 0, parse_values: !csv_as_text, ..Default::default() };
             let (mut wb, report) = gridcraft_xlsx::read_csv_with_report(bytes, &opts).map_err(|e| EngineError::Other(e.to_string()))?;
             rename_first_sheet(&mut wb, name);
-            Ok((wb, report.warnings))
+            let mut warnings = report.warnings;
+            // Text under an Excel package name is a truncated, unsynced or misnamed file far more
+            // often than a CSV; it still opens as text, but says so (#3975).
+            if kind == Some(FileKind::Xlsx) {
+                warnings.insert(0, format!("'{name}' isn't a valid .xlsx package (it is not a ZIP file), so it was read as plain text."));
+            }
+            Ok((wb, warnings))
         }
     }
 }

crates/engine/src/io/tests.rs

@@ -881,3 +881,14 @@ fn the_merge_index_holds_a_bounded_number_of_cells() {
     sh.merges = vec![RangeRef::new(CellRef::new(0, 0), CellRef::new(1_000_000, 0))];
     assert!(merge_index(&sh, rect).is_none());
 }
+
+#[test]
+fn text_named_xlsx_opens_with_a_warning_but_real_csv_does_not() {
+    // #3975: a ".xlsx" that isn't a ZIP package was read as a one-cell text file without a word.
+    let (wb, warnings) = open_bytes("broken.xlsx", b"kein zip\n").unwrap();
+    assert_eq!(wb.sheets.len(), 1);
+    assert!(warnings.iter().any(|w| w.contains("broken.xlsx") && w.contains("isn't a valid .xlsx package")), "{warnings:?}");
+    // Text under an honest name opens as before, with no warning.
+    let (_, warnings) = open_bytes("book.csv", b"a,b\n1,2\n").unwrap();
+    assert!(warnings.is_empty(), "{warnings:?}");
+}