diff --git a/lib/marcel/mime_type.rb b/lib/marcel/mime_type.rb
index 6d7c15e..23da698 100644
--- a/lib/marcel/mime_type.rb
+++ b/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)
diff --git a/test/declared_type_test.rb b/test/declared_type_test.rb
index d5de3d0..d1feeac 100644
--- a/test/declared_type_test.rb
+++ b/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