diff --git a/app/models/optimized_image.rb b/app/models/optimized_image.rb
index 72b37912eed..a344d4f5fc5 100644
--- a/app/models/optimized_image.rb
+++ b/app/models/optimized_image.rb
@@ -139,8 +139,8 @@ class OptimizedImage < ActiveRecord::Base
 
   IM_DECODERS ||= /\A(jpe?g|png|tiff?|bmp|ico|gif)\z/i
 
-  def self.prepend_decoder!(path, ext_path = nil)
-    extension = File.extname(ext_path || path)[1..-1]
+  def self.prepend_decoder!(path, ext_path = nil, opts = nil)
+    extension = File.extname((opts && opts[:filename]) || ext_path || path)[1..-1]
     raise Discourse::InvalidAccess if !extension || !extension.match?(IM_DECODERS)
     "#{extension}:#{path}"
   end
@@ -153,8 +153,8 @@ class OptimizedImage < ActiveRecord::Base
     ensure_safe_paths!(from, to)
 
     # note FROM my not be named correctly
-    from = prepend_decoder!(from, to)
-    to = prepend_decoder!(to, to)
+    from = prepend_decoder!(from, to, opts)
+    to = prepend_decoder!(to, to, opts)
 
     # NOTE: ORDER is important!
     %W{
@@ -190,8 +190,8 @@ class OptimizedImage < ActiveRecord::Base
   def self.crop_instructions(from, to, dimensions, opts = {})
     ensure_safe_paths!(from, to)
 
-    from = prepend_decoder!(from, to)
-    to = prepend_decoder!(to, to)
+    from = prepend_decoder!(from, to, opts)
+    to = prepend_decoder!(to, to, opts)
 
     %W{
       convert
@@ -225,8 +225,8 @@ class OptimizedImage < ActiveRecord::Base
   def self.downsize_instructions(from, to, dimensions, opts = {})
     ensure_safe_paths!(from, to)
 
-    from = prepend_decoder!(from, to)
-    to = prepend_decoder!(to, to)
+    from = prepend_decoder!(from, to, opts)
+    to = prepend_decoder!(to, to, opts)
 
     %W{
       convert
diff --git a/lib/upload_creator.rb b/lib/upload_creator.rb
index f25f7ae677e..3bb50448382 100644
--- a/lib/upload_creator.rb
+++ b/lib/upload_creator.rb
@@ -34,7 +34,13 @@ class UploadCreator
     end
 
     DistributedMutex.synchronize("upload_#{user_id}_#{@filename}") do
-      if FileHelper.is_image?(@filename)
+      # test for image regardless of input
+      @image_info = FastImage.new(@file) rescue nil
+
+      is_image = FileHelper.is_image?(@filename)
+      is_image ||= @image_info && FileHelper.is_image?("test.#{@image_info.type}")
+
+      if is_image
         extract_image_info!
         return @upload if @upload.errors.present?
 
@@ -51,6 +57,7 @@ class UploadCreator
           optimize!        if should_optimize?
         end
 
+        # conversion may have switched the type
         image_type = @image_info.type.to_s
       end
 
@@ -69,17 +76,27 @@ class UploadCreator
       # return the previous upload if any
       return @upload unless @upload.nil?
 
+      fixed_original_filename = nil
+      if is_image
+        # we have to correct original filename here, no choice
+        # otherwise validation will fail and we can not save
+        # TODO decide if we only run the validation on the extension
+        if File.extname(@filename) != ".#{image_type}"
+          fixed_original_filename = "#{@filename}.fixed.#{image_type}"
+        end
+      end
+
       # create the upload otherwise
       @upload = Upload.new
       @upload.user_id           = user_id
-      @upload.original_filename = @filename
+      @upload.original_filename = fixed_original_filename || @filename
       @upload.filesize          = filesize
       @upload.sha1              = sha1
       @upload.url               = ""
       @upload.origin            = @opts[:origin][0...1000] if @opts[:origin]
       @upload.extension         = image_type || File.extname(@filename)[1..10]
 
-      if FileHelper.is_image?(@filename)
+      if is_image
         @upload.width, @upload.height = ImageSizer.resize(*@image_info.size)
       end
 
@@ -101,7 +118,7 @@ class UploadCreator
         end
       end
 
-      if @upload.errors.empty? && FileHelper.is_image?(@filename) && @opts[:type] == "avatar"
+      if @upload.errors.empty? && is_image && @opts[:type] == "avatar"
         Jobs.enqueue(:create_avatar_thumbnails, upload_id: @upload.id, user_id: user_id)
       end
 
@@ -141,7 +158,7 @@ class UploadCreator
 
     OptimizedImage.ensure_safe_paths!(from, to)
 
-    from = OptimizedImage.prepend_decoder!(from)
+    from = OptimizedImage.prepend_decoder!(from, nil, filename: "image.#{@image_info.type}")
     to = OptimizedImage.prepend_decoder!(to)
 
     begin
@@ -229,7 +246,7 @@ class UploadCreator
     path = @file.path
 
     OptimizedImage.ensure_safe_paths!(path)
-    path = OptimizedImage.prepend_decoder!(path)
+    path = OptimizedImage.prepend_decoder!(path, nil, filename: "image.#{@image_info.type}")
 
     Discourse::Utils.execute_command('convert', path, '-auto-orient', path)
 
@@ -243,20 +260,22 @@ class UploadCreator
   def crop!
     max_pixel_ratio = Discourse::PIXEL_RATIOS.max
 
+    filename_with_correct_ext = "image.#{@image_info.type}"
+
     case @opts[:type]
     when "avatar"
       width = height = Discourse.avatar_sizes.max
-      OptimizedImage.resize(@file.path, @file.path, width, height, filename: @filename, allow_animation: allow_animation)
+      OptimizedImage.resize(@file.path, @file.path, width, height, filename: filename_with_correct_ext, allow_animation: allow_animation)
     when "profile_background"
       max_width = 850 * max_pixel_ratio
       width, height = ImageSizer.resize(@image_info.size[0], @image_info.size[1], max_width: max_width, max_height: max_width)
-      OptimizedImage.downsize(@file.path, @file.path, "#{width}x#{height}\>", filename: @filename, allow_animation: allow_animation)
+      OptimizedImage.downsize(@file.path, @file.path, "#{width}x#{height}\>", filename: filename_with_correct_ext, allow_animation: allow_animation)
     when "card_background"
       max_width = 590 * max_pixel_ratio
       width, height = ImageSizer.resize(@image_info.size[0], @image_info.size[1], max_width: max_width, max_height: max_width)
-      OptimizedImage.downsize(@file.path, @file.path, "#{width}x#{height}\>", filename: @filename, allow_animation: allow_animation)
+      OptimizedImage.downsize(@file.path, @file.path, "#{width}x#{height}\>", filename: filename_with_correct_ext, allow_animation: allow_animation)
     when "custom_emoji"
-      OptimizedImage.downsize(@file.path, @file.path, "100x100\>", filename: @filename, allow_animation: allow_animation)
+      OptimizedImage.downsize(@file.path, @file.path, "100x100\>", filename: filename_with_correct_ext, allow_animation: allow_animation)
     end
 
     extract_image_info!
diff --git a/spec/fixtures/images/png_as.jpg b/spec/fixtures/images/png_as.bin
similarity index 100%
rename from spec/fixtures/images/png_as.jpg
rename to spec/fixtures/images/png_as.bin
diff --git a/spec/lib/upload_creator_spec.rb b/spec/lib/upload_creator_spec.rb
index 4238918225d..9c23672eeea 100644
--- a/spec/lib/upload_creator_spec.rb
+++ b/spec/lib/upload_creator_spec.rb
@@ -26,7 +26,7 @@ RSpec.describe UploadCreator do
     end
 
     describe 'when image has the wrong extension' do
-      let(:filename) { "png_as.jpg" }
+      let(:filename) { "png_as.bin" }
       let(:file) { file_from_fixtures(filename) }
 
       it 'should store the upload with the right extension' do
@@ -41,7 +41,7 @@ RSpec.describe UploadCreator do
 
         expect(upload.extension).to eq('png')
         expect(File.extname(upload.url)).to eq('.png')
-        expect(upload.original_filename).to eq('png_as.jpg')
+        expect(upload.original_filename).to eq('png_as.bin.fixed.png')
       end
     end
 
@@ -65,7 +65,7 @@ RSpec.describe UploadCreator do
 
         expect(upload.extension).to eq('jpeg')
         expect(File.extname(upload.url)).to eq('.jpeg')
-        expect(upload.original_filename).to eq('logo.png')
+        expect(upload.original_filename).to eq('logo.png.fixed.jpeg')
       end
     end
   end
diff --git a/spec/models/optimized_image_spec.rb b/spec/models/optimized_image_spec.rb
index beb94b02ff6..0b7fbd63869 100644
--- a/spec/models/optimized_image_spec.rb
+++ b/spec/models/optimized_image_spec.rb
@@ -27,6 +27,32 @@ describe OptimizedImage do
     end
 
     describe '.resize' do
+      it 'should work correctly when extension is bad' do
+
+        original_path = Dir::Tmpname.create(['origin', '.bin']) { nil }
+
+        begin
+          FileUtils.cp "#{Rails.root}/spec/fixtures/images/logo.png", original_path
+
+          # we use "filename" to get the correct extension here, it is more important
+          # then any other param
+
+          OptimizedImage.resize(
+            original_path,
+            original_path,
+            5,
+            5,
+            filename: "test.png"
+          )
+
+          expect(File.read(original_path)).to eq(
+            File.read("#{Rails.root}/spec/fixtures/images/resized.png")
+          )
+        ensure
+          File.delete(original_path) if File.exists?(original_path)
+        end
+      end
+
       it 'should work correctly' do
         tmp_path = "/tmp/resized.png"