Change file to be imported to MS-Excel format

Spreadsheet gem only supports MS-Excel as per described:
https://github.com/zdavatz/spreadsheet#description

Gemfile

@@ -100,3 +100,4 @@ gem "devise-encryptable"
 gem 'tzinfo-data', platforms: %i[mingw mswin x64_mingw jruby]
 gem 'activejob', '~> 5.2.0'
 gem 'ransack_ui', path: 'vendor/gems/ransack_ui-1.3.4' # Vendored until our fix is merged and released
+gem 'spreadsheet'

Gemfile.lock

@@ -334,6 +334,7 @@ GEM
       rainbow (>= 2.2.2, < 4.0)
       ruby-progressbar (~> 1.7)
       unicode-display_width (>= 1.4.0, < 1.7)
+    ruby-ole (1.2.12.2)
     ruby-progressbar (1.10.1)
     rubyzip (2.3.0)
     sass (3.7.4)
@@ -359,6 +360,8 @@ GEM
     sixarm_ruby_unaccent (1.2.0)
     sort_alphabetical (1.1.0)
       unicode_utils (>= 1.2.2)
+    spreadsheet (1.2.6)
+      ruby-ole (>= 1.0)
     sprockets (3.7.2)
       concurrent-ruby (~> 1.0)
       rack (> 1, < 3)
@@ -470,6 +473,7 @@ DEPENDENCIES
   select2-rails
   selenium-webdriver
   simple_form
+  spreadsheet
   sprockets-rails (>= 3.0.0)
   sqlite3 (~> 1.3.13)
   therubyracer

app/models/files/imported_file.rb

@@ -30,7 +30,7 @@ def generate_md5sum
   private
 
   def filetype
-    valid = File.open(filename).type_from_file_command == "text/xml" rescue ""
+    valid = File.open(filename).type_from_file_command == "application/vnd.ms-excel" rescue ""
     if valid == ""
       errors.add(:filename, "no such file")
     end