Simplify pkg mimedb (#6549)

Content-Type resolution can now use a function `TypeByExtension(extension)` 
to resolve to the respective content-type.

pkg/mimedb/db_test.go

@@ -29,3 +29,17 @@ func TestMimeLookup(t *testing.T) {
 		t.Fatalf("Invalid content type are found expected \"false\", got %t", compressible)
 	}
 }
+
+func TestTypeByExtension(t *testing.T) {
+	var contentType string
+	// Test TypeByExtension.
+	contentType = TypeByExtension(".txt")
+	if contentType != "text/plain" {
+		t.Fatalf("Invalid content type are found expected \"text/plain\", got %s", contentType)
+	}
+	// Test non-existent type resolution
+	contentType = TypeByExtension(".abc")
+	if contentType != "application/octet-stream" {
+		t.Fatalf("Invalid content type are found expected \"application/octet-stream\", got %s", contentType)
+	}
+}

pkg/mimedb/resolve-db.go

@@ -0,0 +1,33 @@
+/*
+ * mime-db: Mime Database, (C) 2015, 2016, 2017, 2018 Minio, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package mimedb
+
+import (
+	"strings"
+)
+
+// TypeByExtension resolves the extension to its respective content-type.
+func TypeByExtension(ext string) string {
+	// Set default to "application/octet-stream".
+	var contentType = "application/octet-stream"
+	if ext != "" {
+		if content, ok := DB[strings.ToLower(strings.TrimPrefix(ext, "."))]; ok {
+			contentType = content.ContentType
+		}
+	}
+	return contentType
+}