Handle unknown static file extensions

[danielskeenan]

Fixes #410.

[crow-clang-format]

--- include/crow/http_response.h	(before formatting)
+++ include/crow/http_response.h	(after formatting)
@@ -251,7 +251,9 @@
                     if (mimeType != mime_types.end())
                     {
                         this->add_header("Content-Type", mimeType->second);
-                    } else {
+                    }
+                    else
+                    {
                         this->add_header("Content-Type", "text/plain");
                     }
                 }