So, in your controller you have create/2 where you catch the happy path in the with clause. What I think is happening here is that you are actually getting a changeset error that isn’t being handled. The reason I think this is the case is because of the fallback controller function definition error.
You don’t have an else clause for the with so create/2 is just returning the {:error, changeset} of Repo.insert() in create_image/1. But, {:error, changeset} isn’t a valid %Plug.Conn{} struct – the expected return of your controller functions!
Since you’re not returning a %Plug.Conn{}, Phoenix tries to help you out and call your defined fallback controller ApiAppWeb.FallbackController. You are likely missing the function call(conn, {:error, changeset}) there (you might want to pattern match against the changeset against %Ecto.Changeset{} to avoid a catch all match…)
So you have a couple options to debug from here. You can define the fallback controller call/2 and inspect in there, or put an inspect in your changeset pipeline to see what’s going wrong. (The error is actually in the previous error you posted, at the very end of the line)
I know this is a lot so let me know if you have any questions! Also here’s the documentation on the fallback controller with good examples!
