diff --git a/data/strings/en.ini b/data/strings/en.ini
index f563a33f6..86b11440c 100644
--- a/data/strings/en.ini
+++ b/data/strings/en.ini
@@ -95,6 +95,11 @@ Warning
 <<Do you really want to uninstall '{0}' extension?
 ||&Yes||&No
 END
+unknown_output_file_format_error = <<<END
+Aseprite
+<<Unknown file format '{0}' in output filename
+||&OK
+END
 update_screen_ui_scaling_with_theme_values = <<<END
 Update Screen/UI Scaling
 <<The new theme '{0}' wants to adjust some values for you:
diff --git a/data/widgets/export_file.xml b/data/widgets/export_file.xml
index 190fff5c5..38e2ab7fb 100644
--- a/data/widgets/export_file.xml
+++ b/data/widgets/export_file.xml
@@ -42,7 +42,7 @@
       <hbox cell_hspan="3">
         <boxfiller />
         <hbox homogeneous="true">
-          <button text="@.export" minwidth="60" closewindow="true" id="ok" magnet="true" />
+          <button text="@.export" minwidth="60" id="ok" magnet="true" />
           <button text="@.cancel" minwidth="60" closewindow="true" />
         </hbox>
       </hbox>
diff --git a/laf b/laf
index 8cf772813..b135dbfb5 160000
--- a/laf
+++ b/laf
@@ -1 +1 @@
-Subproject commit 8cf772813063d25116dcf38a5f3a41a34a6a32b8
+Subproject commit b135dbfb5adb32f9b14f85eef1e5fa744d1c553e
diff --git a/src/app/ui/export_file_window.cpp b/src/app/ui/export_file_window.cpp
index 3d75d4d74..dee1eb151 100644
--- a/src/app/ui/export_file_window.cpp
+++ b/src/app/ui/export_file_window.cpp
@@ -11,16 +11,21 @@
 #include "app/ui/export_file_window.h"
 
 #include "app/document.h"
+#include "app/file/file.h"
 #include "app/i18n/strings.h"
 #include "app/ui/layer_frame_comboboxes.h"
 #include "app/ui_context.h"
 #include "base/bind.h"
 #include "base/convert_to.h"
 #include "base/fs.h"
+#include "base/string.h"
 #include "doc/frame_tag.h"
 #include "doc/selected_frames.h"
 #include "doc/site.h"
 #include "fmt/format.h"
+#include "ui/alert.h"
+
+#include <algorithm>
 
 namespace app {
 
@@ -29,8 +34,6 @@ ExportFileWindow::ExportFileWindow(const Document* doc)
   , m_docPref(Preferences::instance().document(doc))
   , m_preferredResize(1)
 {
-  auto& pref = Preferences::instance();
-
   // Is a default output filename in the preferences?
   if (!m_docPref.saveCopy.filename().empty()) {
     setOutputFilename(m_docPref.saveCopy.filename());
@@ -38,8 +41,7 @@ ExportFileWindow::ExportFileWindow(const Document* doc)
   else {
     std::string newFn = base::replace_extension(
       doc->filename(),
-      (doc->sprite()->totalFrames() > 1 ? pref.exportFile.animationDefaultExtension():
-                                          pref.exportFile.imageDefaultExtension()));
+      defaultExtension());
     if (newFn == doc->filename()) {
       newFn = base::join_path(
         base::get_file_path(newFn),
@@ -80,6 +82,7 @@ ExportFileWindow::ExportFileWindow(const Document* doc)
   frames()->Change.connect(base::Bind<void>(&ExportFileWindow::updateAniDir, this));
   forTwitter()->Click.connect(base::Bind<void>(&ExportFileWindow::updateAdjustResizeButton, this));
   adjustResize()->Click.connect(base::Bind<void>(&ExportFileWindow::onAdjustResize, this));
+  ok()->Click.connect(base::Bind<void>(&ExportFileWindow::onOK, this));
 }
 
 bool ExportFileWindow::show()
@@ -200,4 +203,36 @@ void ExportFileWindow::onAdjustResize()
   adjustResize()->parent()->layout();
 }
 
+void ExportFileWindow::onOK()
+{
+  base::paths exts = get_writable_extensions();
+  std::string ext = base::string_to_lower(
+    base::get_file_extension(m_outputFilename));
+
+  // Add default extension to output filename
+  if (std::find(exts.begin(), exts.end(), ext) == exts.end()) {
+    if (ext.empty()) {
+      m_outputFilename =
+        base::replace_extension(m_outputFilename,
+                                defaultExtension());
+    }
+    else {
+      ui::Alert::show(
+        fmt::format(Strings::alerts_unknown_output_file_format_error(), ext));
+      return;
+    }
+  }
+
+  closeWindow(ok());
+}
+
+std::string ExportFileWindow::defaultExtension() const
+{
+  auto& pref = Preferences::instance();
+  if (m_doc->sprite()->totalFrames() > 1)
+    return pref.exportFile.animationDefaultExtension();
+  else
+    return pref.exportFile.imageDefaultExtension();
+}
+
 } // namespace app
diff --git a/src/app/ui/export_file_window.h b/src/app/ui/export_file_window.h
index 13d2df068..00f264a26 100644
--- a/src/app/ui/export_file_window.h
+++ b/src/app/ui/export_file_window.h
@@ -42,6 +42,8 @@ namespace app {
     void updateAniDir();
     void updateAdjustResizeButton();
     void onAdjustResize();
+    void onOK();
+    std::string defaultExtension() const;
 
     const Document* m_doc;
     DocumentPreferences& m_docPref;