[HOT-FIX] Download attachment is failed on iOS (#4405)

model/lib/email/attachment.dart

@@ -29,6 +29,7 @@ class Attachment with EquatableMixin {
   static const String eventICSSubtype = 'ics';
   static const String eventCalendarSubtype = 'calendar';
   static const String applicationRTFType = 'application/rtf';
+  static const String _defaultName = 'unknown-attachment';
 
   final PartId? partId;
   final Id? blobId;
@@ -72,11 +73,17 @@ class Attachment with EquatableMixin {
   }
 
   String generateFileName() {
-    if (name?.isNotEmpty == true) {
-      return name!;
-    } else {
-      return '${blobId?.value}.${type?.subtype}';
-    }
+    final rawName = (name?.trim().isNotEmpty == true)
+        ? name!.trim()
+        : (blobId != null
+            ? (type?.subtype != null
+                ? '${blobId!.value}.${type!.subtype}'
+                : blobId!.value)
+            : _defaultName);
+
+    final sanitized = rawName.replaceAll(RegExp(r'[\\/:*?"<>|]'), '_').trim();
+    // Fall back if sanitized name has no meaningful content (e.g. '???' → '___')
+    return sanitized.contains(RegExp(r'[^_\s]')) ? sanitized : _defaultName;
   }
 
   factory Attachment.fromJson(Map<String, dynamic> json) => _$AttachmentFromJson(json);

model/test/email/attachment_test.dart

@@ -63,5 +63,122 @@ void main() {
         expect(result, 'http://localhost/download/1/some-blob-id/ti%C3%AAu%20%C4%91%E1%BB%81%20attachment?name=ti%C3%AAu%20%C4%91%E1%BB%81%20attachment&type=application%2Foctet-stream');
       });
     });
+
+    group('generateFileName:', () {
+      test(
+        'should return name when name is not empty',
+      () {
+        final attachment = Attachment(
+          blobId: Id('some-blob-id'),
+          name: 'document.pdf',
+          type: MediaType.parse('application/pdf'),
+        );
+
+        expect(attachment.generateFileName(), 'document.pdf');
+      });
+
+      test(
+        'should return name is trimmed when name has leading/trailing spaces',
+      () {
+        final attachment = Attachment(
+          blobId: Id('some-blob-id'),
+          name: '  document.pdf  ',
+          type: MediaType.parse('application/pdf'),
+        );
+
+        expect(attachment.generateFileName(), 'document.pdf');
+      });
+
+      test(
+        'should return blobId with extension when name is null',
+      () {
+        final attachment = Attachment(
+          blobId: Id('some-blob-id'),
+          type: MediaType.parse('image/png'),
+        );
+
+        expect(attachment.generateFileName(), 'some-blob-id.png');
+      });
+
+      test(
+        'should return blobId with extension when name is empty',
+      () {
+        final attachment = Attachment(
+          blobId: Id('some-blob-id'),
+          name: '',
+          type: MediaType.parse('image/png'),
+        );
+
+        expect(attachment.generateFileName(), 'some-blob-id.png');
+      });
+
+      test(
+        'should return blobId with extension when name is only whitespace',
+      () {
+        final attachment = Attachment(
+          blobId: Id('some-blob-id'),
+          name: '   ',
+          type: MediaType.parse('image/jpeg'),
+        );
+
+        expect(attachment.generateFileName(), 'some-blob-id.jpeg');
+      });
+
+      test(
+        'should return blobId without extension when name is null and type is null',
+      () {
+        final attachment = Attachment(
+          blobId: Id('some-blob-id'),
+        );
+
+        expect(attachment.generateFileName(), 'some-blob-id');
+      });
+
+      test(
+        'should return default name when both name and blobId are null',
+      () {
+        final attachment = Attachment(
+          type: MediaType.parse('image/png'),
+        );
+
+        expect(attachment.generateFileName(), 'unknown-attachment');
+      });
+
+      test(
+        'should sanitize name with question mark character',
+      () {
+        final attachment = Attachment(
+          blobId: Id('some-blob-id'),
+          name: 'doc?.pdf',
+          type: MediaType.parse('application/pdf'),
+        );
+
+        expect(attachment.generateFileName(), 'doc_.pdf');
+      });
+
+      test(
+        'should sanitize name with forward slash character',
+      () {
+        final attachment = Attachment(
+          blobId: Id('some-blob-id'),
+          name: 'folder/document.pdf',
+          type: MediaType.parse('application/pdf'),
+        );
+
+        expect(attachment.generateFileName(), 'folder_document.pdf');
+      });
+
+      test(
+        'should return default name when name consists entirely of illegal characters',
+      () {
+        final attachment = Attachment(
+          blobId: Id('some-blob-id'),
+          name: '???',
+          type: MediaType.parse('application/pdf'),
+        );
+
+        expect(attachment.generateFileName(), 'unknown-attachment');
+      });
+    });
   });
 }