Extract target_extname to allow 'html:HTML:EmbedImages'

fix DocSpring#27

When the target format was 'html:HTML:EmbededImages', the current implementation of
the extension extraction logic generated an incorrect file extension of 'html:HTML'.

To address this issue, we have revised the extension extraction logic to correctly
identify and generate the appropriate file extension for the target format.

lib/libreconv.rb

@@ -113,7 +113,12 @@ def escaped_source_path
 
     # @return [String]
     def target_filename
-      File.basename(escaped_source_path, '.*') + '.' + File.basename(@convert_to, ':*')
+      File.basename(escaped_source_path, '.*') + '.' + target_extname
+    end
+
+    # @return [String]
+    def target_extname
+      @convert_to.split(':')[0]
     end
 
     # @raise [IOError] If soffice headless command line tool not found.

spec/libreconv/converter_convert_spec.rb

@@ -10,6 +10,17 @@
       end
     end
 
+    it 'converts a file to the specified target html with EmbedImages options' do
+      create_tmpfile(['', '.html']) do |target_file|
+        fixture_file = fixture_path('docx_with_image.docx')
+        described_class.new(fixture_file, target_file, nil, 'html:HTML:EmbedImages').convert
+
+        converted_file = File.read(target_file)
+        expect(File.size?(target_file)).to be > 0
+        expect(converted_file).to include('data:image/png;base64')
+      end
+    end
+
     Helpers::FILE_TYPES.each do |type|
       it "converts a #{type} to pdf" do
         Dir.mktmpdir do |target_path|