Swap the fixture for "invalid" case from PNG to plain text

h/t @mokagio in #329 (comment)

Author: AliSoftware

lib/fastlane/plugin/wpmreleasetoolkit/helper/ios/ios_l10n_helper.rb

@@ -18,7 +18,12 @@ class L10nHelper
         #         - `nil` if the file does not exist or is neither of those format (e.g. not a `.strings` file at all)
         #
         def self.strings_file_type(path:)
-          format_desc, status = Open3.capture2('file', path)
+          # Start by checking it seems like a valid property-list file (and not e.g. an image or plain text file)
+          _, status = Open3.capture2('/usr/bin/plutil', '-lint', path)
+          return nil unless status.success?
+
+          # If it is a valid property-list file, determine the actual format used
+          format_desc, status = Open3.capture2('/usr/bin/file', path)
           return nil unless status.success?
 
           case format_desc

spec/ios_l10n_helper_spec.rb

@@ -33,9 +33,9 @@ def file_encoding(path)
     end
 
     it 'returns nil on file that is not a strings file at all' do
-      png_fixture = fixture('not-a-strings-file.png')
-      expect(File).to exist(png_fixture)
-      expect(described_class.strings_file_type(path: '/invalid-path')).to be_nil
+      invalid_fixture = fixture('invalid-file.strings')
+      expect(File).to exist(invalid_fixture)
+      expect(described_class.strings_file_type(path: invalid_fixture)).to be_nil
     end
   end
 
@@ -117,7 +117,7 @@ def file_encoding(path)
     end
 
     it 'raises if trying to parse an invalid strings file' do
-      file = fixture('not-a-strings-file.png')
+      file = fixture('invalid-file.strings')
       expect do
         described_class.read_strings_file_as_hash(path: file)
       end.to raise_exception(RuntimeError)

spec/test-data/translations/ios_l10n_helper/invalid-file.strings

@@ -0,0 +1,3 @@
+Contrary to what the extension of this file suggests, this is not a `.strings` file.
+Instead, this is just a regular, plain text file, which should be detected as invalid
+if provided as an input to `Ios::L10nHelper` methods.