diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7e09537..65208ed 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,6 +1,8 @@
 ## [Unreleased]
 =======
-* no unreleased changes *
+
+### Added
+* Handling excel files with incorrect file extension
 
 ## 11.2.0 / 2024-04-10
 ### Added
diff --git a/lib/ndr_import/file/excel.rb b/lib/ndr_import/file/excel.rb
index cd76555..94a9ca6 100644
--- a/lib/ndr_import/file/excel.rb
+++ b/lib/ndr_import/file/excel.rb
@@ -20,7 +20,10 @@ class Excel < Base
       def tables
         return enum_for(:tables) unless block_given?
 
-        workbook = load_workbook(@filename)
+        # Create a new file using the given format as the extension, if a format is provided
+        path = @format.present? ? create_file_with_correct_extension : @filename
+
+        workbook = load_workbook(path)
         workbook.sheets.each do |sheet_name|
           yield sheet_name, excel_rows(workbook, sheet_name)
         end
@@ -91,11 +94,7 @@ def load_workbook(path)
         when '.xls'
           Roo::Excel.new(SafeFile.safepath_to_string(path))
         when '.xlsm', '.xlsx'
-          if @options['file_password']
-            Roo::Excelx.new(StringIO.new(decrypted_file_string(path, @options['file_password'])))
-          else
-            Roo::Excelx.new(SafeFile.safepath_to_string(path))
-          end
+          load_xlsm_xlsx(path)
         else
           raise "Received file path with unexpected extension #{SafeFile.extname(path)}"
         end
@@ -107,13 +106,42 @@ def load_workbook(path)
         # so we create a duplicate file in xlsx extension
         raise e.message unless /(.*)\.xls$/.match(path)
 
+        load_workbook create_amended_xlsx_from(path)
+      rescue RuntimeError, ::Zip::Error => e
+        raise ["Unable to read the file '#{path}'", e.message].join('; ')
+      ensure
+        SafeFile.delete(path) if @remove_updated_extension_file
+      end
+
+      def load_xlsm_xlsx(path)
+        if @options['file_password']
+          Roo::Excelx.new(StringIO.new(decrypted_file_string(path, @options['file_password'])))
+        else
+          Roo::Excelx.new(SafeFile.safepath_to_string(path))
+        end
+      end
+
+      def create_amended_xlsx_from(path)
         new_file_name = SafeFile.basename(path).gsub(/(.*)\.xls$/, '\1_amend.xlsx')
         new_file_path = SafeFile.dirname(path).join(new_file_name)
         copy_file(path, new_file_path)
 
-        load_workbook(new_file_path)
-      rescue RuntimeError, ::Zip::Error => e
-        raise ["Unable to read the file '#{path}'", e.message].join('; ')
+        new_file_path
+      end
+
+      def create_file_with_correct_extension
+        # We shouldn't attempt to change one excel format to another
+        return @filename if %w[.xls .xlsx .xlsm].include? SafeFile.extname(@filename).downcase
+
+        new_file_name = SafeFile.basename(@filename).sub(/\..+\z/, ".#{@format}")
+        new_file_path = SafeFile.dirname(@filename).join(new_file_name)
+        return @filename if ::File.exist?(new_file_path)
+
+        copy_file(@filename, new_file_path)
+        # Flag the newly made file for deletion once we've finished with it
+        @remove_updated_extension_file = true
+
+        new_file_path
       end
 
       # Note that this method can produce insecure calls. All callers must protect
diff --git a/test/file/excel_test.rb b/test/file/excel_test.rb
index 92836b6..3c49642 100644
--- a/test/file/excel_test.rb
+++ b/test/file/excel_test.rb
@@ -129,6 +129,37 @@ def setup
 
         SafeFile.delete @permanent_test_files.join('txt_file_xls_extension_amend.xlsx')
       end
+
+      test 'should use format provided to read excel file with incorrect file extension' do
+        file_path = @permanent_test_files.join('sample_xlsx_with_dat_extension.dat')
+        handler = NdrImport::File::Excel.new(file_path, 'xlsx')
+        handler.tables.each do |tablename, sheet|
+          assert_equal 'Sheet1', tablename
+          assert_instance_of Enumerator, sheet
+          assert_equal %w[1A 1B], sheet.first
+        end
+      end
+
+      test 'should read xlsx file when an xls format provided incorrectly' do
+        file_path = @permanent_test_files.join('sample_xlsx_with_dat_extension.dat')
+        handler = NdrImport::File::Excel.new(file_path, 'xls')
+        handler.tables.each do |tablename, sheet|
+          assert_equal 'Sheet1', tablename
+          assert_instance_of Enumerator, sheet
+          assert_equal %w[1A 1B], sheet.first
+        end
+
+        # Existing logic doesn't remove the _amend.xlsx file
+        SafeFile.delete @permanent_test_files.join('sample_xlsx_with_dat_extension_amend.xlsx')
+      end
+
+      test 'should fail to load if format provided is not an excel extension' do
+        file_path = @permanent_test_files.join('sample_xlsx_with_dat_extension.dat')
+        handler   = NdrImport::File::Excel.new(file_path, 'cabbage')
+        exception = assert_raises(RuntimeError) { handler.tables.to_a }
+
+        assert_match(/Received file path with unexpected extension \.cabbage/, exception.message)
+      end
     end
   end
 end
diff --git a/test/resources/sample_xlsx_with_dat_extension.dat b/test/resources/sample_xlsx_with_dat_extension.dat
new file mode 100644
index 0000000..d046e72
Binary files /dev/null and b/test/resources/sample_xlsx_with_dat_extension.dat differ