diff --git a/lib/remote_storage/rest_provider.rb b/lib/remote_storage/rest_provider.rb
index 7447c53..740c053 100644
--- a/lib/remote_storage/rest_provider.rb
+++ b/lib/remote_storage/rest_provider.rb
@@ -118,6 +118,9 @@ def get_directory_listing(user, directory)
     end
 
     def put_data(user, directory, key, data, content_type)
+      # Do not try to perform the PUT request when the Content-Type does not
+      # look like a MIME type
+      server.halt 415 unless content_type.match(/^.+\/.+/i)
       server.halt 400 if server.env["HTTP_CONTENT_RANGE"]
       server.halt 409, "Conflict" if has_name_collision?(user, directory, key)
 
diff --git a/spec/shared_examples.rb b/spec/shared_examples.rb
index 9255f9c..30c44ae 100644
--- a/spec/shared_examples.rb
+++ b/spec/shared_examples.rb
@@ -232,6 +232,16 @@ def storage_class
           _(last_response.body).must_equal "Precondition Failed"
         end
       end
+
+      describe "Content-Type" do
+        it "must be in the type/subtype format" do
+          header "Content-Type", "text"
+
+          put "/phil/food/invalid_content_type", "invalid"
+
+          _(last_response.status).must_equal 415
+        end
+      end
     end
 
   end