swap ruby-filemagic for file command

Gemfile

@@ -61,8 +61,6 @@ gem "auto_strip_attributes"
 # Use sidekiq for background processing
 gem "sidekiq"
 
-gem "ruby-filemagic"
-
 group :development, :test do
   # Check gems for known vulnerabilities
   gem "bundler-audit"

Gemfile.lock

@@ -370,7 +370,6 @@ GEM
       rubocop (~> 1.0)
     rubocop-rspec (2.7.0)
       rubocop (~> 1.19)
-    ruby-filemagic (0.7.3)
     ruby-progressbar (1.11.0)
     ruby2_keywords (0.0.5)
     rubyzip (2.3.2)
@@ -476,7 +475,6 @@ DEPENDENCIES
   rubocop-govuk (= 4.3.0)
   rubocop-performance
   rubocop-rails
-  ruby-filemagic
   selenium-webdriver
   sentry-rails
   sentry-ruby

app/models/forms/bulk_upload_lettings/upload_your_file.rb

@@ -1,3 +1,5 @@
+require "shellwords"
+
 module Forms
   module BulkUploadLettings
     class UploadYourFile
@@ -55,7 +57,10 @@ module Forms
       def validate_file_is_csv
         return unless file
 
-        unless FileMagic.new(FileMagic::MAGIC_MIME).file(file.path).include?("text/csv")
+        argv = %W[file --mime-type -- #{file.path}]
+        output = `#{argv.shelljoin}`
+
+        unless output.include?("text/csv")
           errors.add(:file, :not_csv)
         end
       end

app/models/forms/bulk_upload_sales/upload_your_file.rb

@@ -1,3 +1,5 @@
+require "shellwords"
+
 module Forms
   module BulkUploadSales
     class UploadYourFile
@@ -55,7 +57,10 @@ module Forms
       def validate_file_is_csv
         return unless file
 
-        unless FileMagic.new(FileMagic::MAGIC_MIME).file(file.path).include?("text/csv")
+        argv = %W[file --mime-type -- #{file.path}]
+        output = `#{argv.shelljoin}`
+
+        unless output.include?("text/csv")
           errors.add(:file, :not_csv)
         end
       end