diff --git a/toolkit/mozapps/downloads/HelperAppDlg.jsm b/toolkit/mozapps/downloads/HelperAppDlg.jsm
index 9042007f3d57..df3f7d66dbaf 100644
--- a/toolkit/mozapps/downloads/HelperAppDlg.jsm
+++ b/toolkit/mozapps/downloads/HelperAppDlg.jsm
@@ -461,29 +461,6 @@ nsUnknownContentTypeDialog.prototype = {
       validatedFile = aLocalFolder;
     }
 
-    if (AppConstants.platform == "win") {
-      let ext;
-      try {
-        // We can fail here if there's no primary extension set
-        ext = "." + this.mLauncher.MIMEInfo.primaryExtension;
-      } catch (e) {}
-
-      // Append a file extension if it's an executable that doesn't have one
-      // but make sure we actually have an extension to add
-      let leaf = validatedFile.leafName;
-      if (
-        ext &&
-        !leaf.toLowerCase().endsWith(ext.toLowerCase()) &&
-        validatedFile.isExecutable()
-      ) {
-        validatedFile.remove(false);
-        aLocalFolder.leafName = leaf + ext;
-        if (!aAllowExisting) {
-          validatedFile = DownloadPaths.createNiceUniqueFile(aLocalFolder);
-        }
-      }
-    }
-
     return validatedFile;
   },
 
diff --git a/toolkit/mozapps/downloads/tests/browser/browser.ini b/toolkit/mozapps/downloads/tests/browser/browser.ini
index be4b5b33ce5c..da1cf956e027 100644
--- a/toolkit/mozapps/downloads/tests/browser/browser.ini
+++ b/toolkit/mozapps/downloads/tests/browser/browser.ini
@@ -6,6 +6,7 @@ support-files =
   unknownContentType_dialog_layout_data.txt^headers^
   head.js
 
+[browser_save_wrongextension.js]
 [browser_unknownContentType_blob.js]
 [browser_unknownContentType_delayedbutton.js]
 skip-if =
diff --git a/toolkit/mozapps/downloads/tests/browser/browser_save_wrongextension.js b/toolkit/mozapps/downloads/tests/browser/browser_save_wrongextension.js
new file mode 100644
index 000000000000..00099a0078da
--- /dev/null
+++ b/toolkit/mozapps/downloads/tests/browser/browser_save_wrongextension.js
@@ -0,0 +1,99 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+let url =
+  "data:text/html,<a id='link' href='http://localhost:8000/thefile.js'>Link</a>";
+
+let MockFilePicker = SpecialPowers.MockFilePicker;
+MockFilePicker.init(window);
+
+let httpServer = null;
+
+add_task(async function test() {
+  const { HttpServer } = ChromeUtils.import(
+    "resource://testing-common/httpd.js"
+  );
+
+  httpServer = new HttpServer();
+  httpServer.start(8000);
+
+  function handleRequest(aRequest, aResponse) {
+    aResponse.setStatusLine(aRequest.httpVersion, 200);
+    aResponse.setHeader("Content-Type", "text/plain");
+    aResponse.write("Some Text");
+  }
+  httpServer.registerPathHandler("/thefile.js", handleRequest);
+
+  let tab = await BrowserTestUtils.openNewForegroundTab(gBrowser, url);
+
+  let popupShownPromise = BrowserTestUtils.waitForEvent(document, "popupshown");
+
+  await BrowserTestUtils.synthesizeMouseAtCenter(
+    "#link",
+    { type: "contextmenu", button: 2 },
+    gBrowser.selectedBrowser
+  );
+  await popupShownPromise;
+
+  let tempDir = createTemporarySaveDirectory();
+  let destFile;
+
+  MockFilePicker.displayDirectory = tempDir;
+  MockFilePicker.showCallback = fp => {
+    let fileName = fp.defaultString;
+    destFile = tempDir.clone();
+    destFile.append(fileName);
+
+    MockFilePicker.setFiles([destFile]);
+    MockFilePicker.filterIndex = 1; // kSaveAsType_URL
+  };
+
+  let transferCompletePromise = new Promise(resolve => {
+    mockTransferCallback = resolve;
+    mockTransferRegisterer.register();
+  });
+
+  registerCleanupFunction(function() {
+    mockTransferRegisterer.unregister();
+    tempDir.remove(true);
+  });
+
+  document.getElementById("context-savelink").doCommand();
+
+  let contextMenu = document.getElementById("contentAreaContextMenu");
+  let popupHiddenPromise = BrowserTestUtils.waitForEvent(
+    document,
+    "popuphidden"
+  );
+  contextMenu.hidePopup();
+  await popupHiddenPromise;
+
+  await transferCompletePromise;
+
+  is(destFile.leafName, "thefile.js", "filename extension is not modified");
+
+  BrowserTestUtils.removeTab(tab);
+});
+
+add_task(async () => {
+  MockFilePicker.cleanup();
+  await new Promise(resolve => httpServer.stop(resolve));
+});
+
+/* import-globals-from ../../../../../toolkit/content/tests/browser/common/mockTransfer.js */
+Services.scriptloader.loadSubScript(
+  "chrome://mochitests/content/browser/toolkit/content/tests/browser/common/mockTransfer.js",
+  this
+);
+
+function createTemporarySaveDirectory() {
+  let saveDir = Services.dirsvc.get("TmpD", Ci.nsIFile);
+  saveDir.append("testsavedir");
+  if (!saveDir.exists()) {
+    saveDir.create(Ci.nsIFile.DIRECTORY_TYPE, 0o755);
+  }
+  return saveDir;
+}
diff --git a/uriloader/exthandler/tests/mochitest/browser_save_filenames.js b/uriloader/exthandler/tests/mochitest/browser_save_filenames.js
index 679e1c1efca9..ea46fa5a5777 100644
--- a/uriloader/exthandler/tests/mochitest/browser_save_filenames.js
+++ b/uriloader/exthandler/tests/mochitest/browser_save_filenames.js
@@ -202,14 +202,12 @@ function getItems(parentid) {
           elem.localName == "img" && elem.dataset.nodrag != "true";
         let unknown = elem.dataset.unknown;
         let noattach = elem.dataset.noattach;
-        let winexeext = elem.dataset.winexeext;
         elements.push({
           draggable,
           unknown,
           filename,
           url,
           noattach,
-          winexeext,
         });
         elem = elem.nextElementSibling;
       }
@@ -588,16 +586,6 @@ add_task(async function save_links() {
     let filename = PathUtils.filename(download.target.path);
 
     let expectedFilename = expectedItems[idx].filename;
-    if (AppConstants.platform == "win") {
-      // On Windows, an extension is added to executable files when saving as
-      // an attachment to avoid the file looking like an executable. This is
-      // done in validateLeafName in HelperAppDlg.jsm.
-      // XXXndeakin should we do this for all save mechanisms?
-      if (expectedItems[idx].winexeext) {
-        expectedFilename += "." + expectedItems[idx].winexeext;
-      }
-    }
-
     // Use checkShortenedFilename to check long filenames.
     if (expectedItems[idx].filename.length > 240) {
       ok(
diff --git a/uriloader/exthandler/tests/mochitest/save_filenames.html b/uriloader/exthandler/tests/mochitest/save_filenames.html
index 583ba55ba072..0d7004515a9b 100644
--- a/uriloader/exthandler/tests/mochitest/save_filenames.html
+++ b/uriloader/exthandler/tests/mochitest/save_filenames.html
@@ -86,14 +86,13 @@
 <!-- simple script -->
 <script id="i22" src="http://localhost:8000/save_filename.sjs?type=js&filename=script1.js" data-filename="script1.js"></script>
 
-<!-- script with invalid extension. Windows doesn't have an association for application/x-javascript
-     so doesn't handle it. -->
+<!-- script with invalid extension. -->
 <script id="i23" src="http://localhost:8000/save_filename.sjs?type=js&filename=script2.exe"
-        data-filename="script2.exe" data-winexeext="js"></script>
+        data-filename="script2.exe"></script>
 
 <!-- script with escaped characters -->
 <script id="i24" src="http://localhost:8000/save_filename.sjs?type=js&filename=script%20%33.exe"
-        data-filename="script 3.exe" data-winexeext="js"></script>
+        data-filename="script 3.exe"></script>
 
 <!-- script with long filename -->
 <script id="i25" src="http://localhost:8000/save_filename.sjs?type=js&filename=script123456789script123456789script123456789script123456789script123456789script123456789script123456789script123456789script123456789script123456789script123456789script123456789script123456789script123456789script123456789script123456789script123456789script123456789.js"
@@ -153,11 +152,11 @@
 
 <!-- script where url filename has no extension and invalid extension in content disposition filename -->
 <script id="i41" src="http://localhost:8000/base?type=js&filename=script5.exe"
-        data-filename="script5.exe" data-winexeext="js"></script>
+        data-filename="script5.exe"></script>
 
 <!-- script where url filename has no extension and escaped characters in content disposition filename-->
 <script id="i42" src="http://localhost:8000/base?type=js&filename=script%20%36.exe"
-        data-filename="script 6.exe" data-winexeext="js"></script>
+        data-filename="script 6.exe"></script>
 
 <!-- text where filename is present -->
 <img id="i43" src="http://localhost:8000/getdata.png?type=text&filename=readme.txt"
@@ -202,11 +201,9 @@
 <img id="i53" src="http://localhost:8000/save_filename.sjs?&filename=notype.png"
      data-nodrag="true" data-filename="notype.png">
 
-<!-- no content type specified. Note that on Windows, .txt is
-     appended when saving as an attachment. This is special-cased
-     in browser_save_filenames.js. -->
+<!-- no content type specified. -->
 <img id="i54" src="http://localhost:8000/save_filename.sjs?&filename=notypebin.exe"
-     data-nodrag="true" data-filename="notypebin.exe" data-winexeext="txt">
+     data-nodrag="true" data-filename="notypebin.exe">
 
 <!-- extension contains invalid characters -->
 <img id="i55" src="http://localhost:8000/save_filename.sjs?type=png&filename=extinvalid.a?*"
@@ -261,7 +258,7 @@
 
 <!-- simple zip file with differing extension -->
 <object id="i68" data="http://localhost:8000/save_filename.sjs?type=zip&filename=simple.jar" data-filename="simple.jar"
-        data-unknown="true" data-winexeext="zip"></object>
+        data-unknown="true"></object>
 
 <!-- simple zip file with no extension -->
 <object id="i69" data="http://localhost:8000/save_filename.sjs?type=zip&filename=simplepack" data-filename="simplepack.zip"