diff --git a/app/models/attachment.rb b/app/models/attachment.rb
index e862ede7..9c65cec1 100644
--- a/app/models/attachment.rb
+++ b/app/models/attachment.rb
@@ -17,19 +17,19 @@ class Attachment < ApplicationRecord
   validates_attachment_content_type :file, content_type: Attachable.allowed_types
 
   def image?
-    Attachable.image_types.include(file.instance.file_content_type)
+    Attachable.image_types.include?(file.instance.file_content_type)
   end
 
   def audio?
-    Attachable.audio_types.include(file.instance.file_content_type)
+    Attachable.audio_types.include?(file.instance.file_content_type)
   end
 
   def text?
-    Attachable.text_types.include(file.instance.file_content_type)
+    Attachable.text_types.include?(file.instance.file_content_type)
   end
 
   def pdf?
-    Attachable.pdf_types.include(file.instance.file_content_type)
+    Attachable.pdf_types.include?(file.instance.file_content_type)
   end
 
   def document?
diff --git a/app/models/concerns/attachable.rb b/app/models/concerns/attachable.rb
index dbcc2a2a..87250a09 100644
--- a/app/models/concerns/attachable.rb
+++ b/app/models/concerns/attachable.rb
@@ -36,7 +36,7 @@ module Attachable
     end
 
     def text_types
-      ['text/plain', 'text/markdown']
+      ['text/plain']
     end
 
     def pdf_types