Update error messaging to indicate file extension rather than MIME type

app/models/user.rb

@@ -2112,7 +2112,7 @@ class User < ApplicationRecord
   def check_username_format
     return if username.blank? || Mime::EXTENSION_LOOKUP.keys.none? { |type| username.end_with?(".#{type}") }
 
-    errors.add(:username, _('ending with MIME type format is not allowed.'))
+    errors.add(:username, _('ending with a file extension is not allowed.'))
   end
 
   def groups_with_developer_maintainer_project_access

locale/gitlab.pot

@@ -39780,7 +39780,7 @@ msgstr ""
 msgid "encrypted: needs to be a :required, :optional or :migrating!"
 msgstr ""
 
-msgid "ending with MIME type format is not allowed."
+msgid "ending with a file extension is not allowed."
 msgstr ""
 
 msgid "entries cannot be larger than 255 characters"

spec/models/user_spec.rb

@@ -401,7 +401,7 @@ RSpec.describe User do
           user = build(:user, username: "test.#{type}")
 
           expect(user).not_to be_valid
-          expect(user.errors.full_messages).to include('Username ending with MIME type format is not allowed.')
+          expect(user.errors.full_messages).to include('Username ending with a file extension is not allowed.')
           expect(build(:user, username: "test#{type}")).to be_valid
         end
       end