The other day, I was building a Rails API for a third-party service to use. My tests were all green, but when they started integrating, things went sideways. Some attributes were just vanishing from the request payload on my Rails end. What gives?

Turns out, it was all about the Content-Type header. They were sending requests with Content-Type: application/vnd.ims.lis.v1.score+json. Now, Rails is smart, it knows how to handle common types like application/json. But this one? It was like a foreign language to Rails – it just treated the whole payload as plain text and didn’t parse the JSON properly.

Quick aside: MIME types (or media types) are basically labels that tell systems what kind of data they’re dealing with. Think text/html for web pages or image/jpeg for pictures. They’re essential for things to work smoothly, especially with APIs.

So, to fix my missing payload problem, I had to teach Rails this new MIME type. I added this snippet to my config/initializers/mime_types.rb file:

json_mime_type_synonyms = %w[application/vnd.ims.lis.v1.score+json]
Mime::Type.register("application/json", :json, json_mime_type_synonyms)

Basically, I told Rails, “Hey, when you see this weird application/vnd.ims.lis.v1.score+json thing, treat it like good old application/json and parse the JSON inside.”

Lesson learned: MIME types are important, even if they look like alphabet soup sometimes. Rails doesn’t know everything, so you might need to add custom types to your mime_types.rb if you’re dealing with APIs and external services. It’s a small tweak that can save you a lot of debugging headaches down the road.