diff --git a/src/Application/Interfaces/Services/IBookService.cs b/src/Application/Interfaces/Services/IBookService.cs
index 8bd4ad3..0dce6d4 100644
--- a/src/Application/Interfaces/Services/IBookService.cs
+++ b/src/Application/Interfaces/Services/IBookService.cs
@@ -16,4 +16,5 @@ public interface IBookService
     Task<Stream> GetBookCover(string email, Guid guid);
     Task DeleteBookCover(string email, Guid guid);
     Task<string> GetFormatForBook(string email, Guid guid);
+    Task<string> GetExtensionForBook(string email, Guid guid);
 }
\ No newline at end of file
diff --git a/src/Application/Services/BookService.cs b/src/Application/Services/BookService.cs
index ac0f806..719a424 100644
--- a/src/Application/Services/BookService.cs
+++ b/src/Application/Services/BookService.cs
@@ -255,6 +255,19 @@ public async Task<string> GetFormatForBook(string email, Guid guid)
         return book.Format;
     }
 
+    public async Task<string> GetExtensionForBook(string email, Guid guid)
+    {
+        var user = await _userRepository.GetAsync(email, trackChanges: true);
+        var book = user.Books.SingleOrDefault(book => book.BookId == guid);
+        if (book == null)
+        {
+            const string message = "No book with this id exists";
+            throw new CommonErrorException(404, message, 4);
+        }
+
+        return book.Extension;
+    }
+
     public async Task ChangeBookCover(string email, Guid guid, MultipartReader reader)
     {
         var user = await _userRepository.GetAsync(email, trackChanges: true);
diff --git a/src/Presentation/Controllers/BookController.cs b/src/Presentation/Controllers/BookController.cs
index 1c36806..fccd72a 100644
--- a/src/Presentation/Controllers/BookController.cs
+++ b/src/Presentation/Controllers/BookController.cs
@@ -71,9 +71,14 @@ public async Task<ActionResult> GetBookBinaryData(Guid guid)
             var email = HttpContext.User.Identity!.Name;
             var stream = await _bookService.GetBookBinaryData(email, 
                                                           guid);
-            
+            // This is added for legacy purposes. Previously the client depended on getting the format, but newer
+            // clients use the extension. We keep it to keep the old clients working.
             Response.Headers.Add("Format",
                                  await _bookService.GetFormatForBook(email, guid));
+            
+            Response.Headers.Add("Extension",
+                await _bookService.GetExtensionForBook(email, guid));
+            
             return File(stream, "application/octet-stream");
         }
         catch (CommonErrorException e)