diff --git a/app/models/attachment.rb b/app/models/attachment.rb
index b1d7ae1942c3..f1eb8ad9cae3 100644
--- a/app/models/attachment.rb
+++ b/app/models/attachment.rb
@@ -163,8 +163,12 @@ def is_pdf?
     content_type == "application/pdf"
   end
 
+  def is_html?
+    content_type == "text/html"
+  end
+
   def is_text?
-    content_type.match?(/\Atext\/.+/)
+    content_type.match?(/\Atext\/.+/) && !is_html?
   end
 
   def is_diff?
diff --git a/spec/models/attachment_spec.rb b/spec/models/attachment_spec.rb
index cb061fcbab00..91b249efa187 100644
--- a/spec/models/attachment_spec.rb
+++ b/spec/models/attachment_spec.rb
@@ -295,6 +295,17 @@
       end
     end
 
+    describe "for an HTML file" do
+      let(:attachment) { described_class.new }
+
+      it "assumes it not to be inlineable" do
+        attachment.content_type = "text/html"
+        expect(attachment).to be_is_html
+        expect(attachment).not_to be_is_text
+        expect(attachment).not_to be_inlineable
+      end
+    end
+
     describe "for a binary file" do
       before { binary_attachment.save! }