diff --git a/dom/file/Blob.cpp b/dom/file/Blob.cpp
index 532468f8d69c..6b6b3910a795 100644
--- a/dom/file/Blob.cpp
+++ b/dom/file/Blob.cpp
@@ -189,11 +189,16 @@ Blob::GetType(nsAString &aType)
 already_AddRefed<Blob>
 Blob::Slice(const Optional<int64_t>& aStart,
             const Optional<int64_t>& aEnd,
-            const nsAString& aContentType,
+            const Optional<nsAString>& aContentType,
             ErrorResult& aRv)
 {
+  nsAutoString contentType;
+  if (aContentType.WasPassed()) {
+    contentType = aContentType.Value();
+  }
+
   RefPtr<BlobImpl> impl =
-    mImpl->Slice(aStart, aEnd, aContentType, aRv);
+    mImpl->Slice(aStart, aEnd, contentType, aRv);
   if (aRv.Failed()) {
     return nullptr;
   }
diff --git a/dom/file/Blob.h b/dom/file/Blob.h
index a0c881944cb0..a62e5c95db97 100644
--- a/dom/file/Blob.h
+++ b/dom/file/Blob.h
@@ -122,7 +122,7 @@ public:
 
   already_AddRefed<Blob> Slice(const Optional<int64_t>& aStart,
                                const Optional<int64_t>& aEnd,
-                               const nsAString& aContentType,
+                               const Optional<nsAString>& aContentType,
                                ErrorResult& aRv);
 
   size_t GetAllocationSize() const;
diff --git a/dom/file/FileReader.h b/dom/file/FileReader.h
index d330ae9202ee..d8b9c2a1897b 100644
--- a/dom/file/FileReader.h
+++ b/dom/file/FileReader.h
@@ -72,9 +72,14 @@ public:
     ReadFileContent(aBlob, EmptyString(), FILE_AS_ARRAYBUFFER, aRv);
   }
 
-  void ReadAsText(Blob& aBlob, const nsAString& aLabel, ErrorResult& aRv)
+  void ReadAsText(Blob& aBlob, const Optional<nsAString>& aLabel,
+                  ErrorResult& aRv)
   {
-    ReadFileContent(aBlob, aLabel, FILE_AS_TEXT, aRv);
+    if (aLabel.WasPassed()) {
+      ReadFileContent(aBlob, aLabel.Value(), FILE_AS_TEXT, aRv);
+    } else {
+      ReadFileContent(aBlob, EmptyString(), FILE_AS_TEXT, aRv);
+    }
   }
 
   void ReadAsDataURL(Blob& aBlob, ErrorResult& aRv)
diff --git a/dom/webidl/Blob.webidl b/dom/webidl/Blob.webidl
index 07e96527b2d6..94812cd89c65 100644
--- a/dom/webidl/Blob.webidl
+++ b/dom/webidl/Blob.webidl
@@ -27,7 +27,7 @@ interface Blob {
   [Throws]
   Blob slice([Clamp] optional long long start,
              [Clamp] optional long long end,
-             optional DOMString contentType = "");
+             optional DOMString contentType);
 };
 
 enum EndingTypes { "transparent", "native" };
diff --git a/dom/webidl/FileReader.webidl b/dom/webidl/FileReader.webidl
index 710f0c7b366b..1e79ba09fd9d 100644
--- a/dom/webidl/FileReader.webidl
+++ b/dom/webidl/FileReader.webidl
@@ -4,7 +4,7 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/.
  *
  * The origin of this IDL file is
- * http://dev.w3.org/2006/webapi/FileAPI/#dfn-filereader
+ * https://w3c.github.io/FileAPI/#APIASynch
  *
  * Copyright © 2013 W3C® (MIT, ERCIM, Keio, Beihang), All Rights Reserved. W3C
  * liability, trademark and document use rules apply.
@@ -17,7 +17,9 @@ interface FileReader : EventTarget {
   [Throws]
   void readAsArrayBuffer(Blob blob);
   [Throws]
-  void readAsText(Blob blob, optional DOMString label = "");
+  void readAsBinaryString(Blob filedata);
+  [Throws]
+  void readAsText(Blob blob, optional DOMString label);
   [Throws]
   void readAsDataURL(Blob blob);
 
@@ -46,9 +48,3 @@ interface FileReader : EventTarget {
   attribute EventHandler onerror;
   attribute EventHandler onloadend;
 };
-
-// Mozilla extensions.
-partial interface FileReader {
-  [Throws]
-  void readAsBinaryString(Blob filedata);
-};