Regression fix: binary declared type should fall back to filename extension type (#99)

The application/octet-stream content type is treated as a default rather
than a specific declaration. We should check the filename extension for
the more specific type in this case. If there's no extension, the type
falls back to binary anyway.

Fixes regression in 1.0.2 -> 1.0.3 introduced by #94.

Author: jeremy

lib/marcel/mime_type.rb

@@ -60,7 +60,12 @@ def for_extension(extension)
         end
 
         def for_declared_type(declared_type)
-          parse_media_type(declared_type)
+          type = parse_media_type(declared_type)
+
+          # application/octet-stream is treated as an undeclared/missing type,
+          # allowing the type to be inferred from the filename. If there's no
+          # filename extension, then the type falls back to binary anyway.
+          type unless type == BINARY
         end
 
         def with_io(pathname_or_io, &block)

test/declared_type_test.rb

@@ -2,11 +2,16 @@
 require 'rack'
 
 class Marcel::MimeType::DeclaredTypeTest < Marcel::TestCase
-  test "returns declared type as last resort" do
-    assert_equal "text/html", Marcel::MimeType.for(name: "unrecognisable", declared_type: "text/html")
+  test "prefers declared type over filename extension" do
+    assert_equal "text/html", Marcel::MimeType.for(name: "file.txt", declared_type: "text/html")
   end
 
-  test "returns application/octet-stream if declared type empty or unrecognised" do
+  test "prefers filename extension over binary type" do
+    assert_equal "text/plain", Marcel::MimeType.for(name: "file.txt", declared_type: "application/octet-stream")
+  end
+
+  test "defaults to binary if declared type is unrecognized" do
+    assert_equal "application/octet-stream", Marcel::MimeType.for(declared_type: nil)
     assert_equal "application/octet-stream", Marcel::MimeType.for(declared_type: "")
     assert_equal "application/octet-stream", Marcel::MimeType.for(declared_type: "unrecognised")
   end