Challenge 1: Fix the Upload Command — Possible Solution
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WHAT'S WRONG: -d "file=@photo.jpg" does NOT upload the file's actual
contents. -d treats its argument as a literal string (or, at most,
form-urlencoded data) — it does NOT interpret the @photo.jpg syntax as
"read this file's contents," the way -F does. The server receives the
literal text "file=@photo.jpg" as form-urlencoded body data, not the
photo's binary contents wrapped in a proper multipart body — which
explains exactly why the file arrives corrupted or unusable: there
never was a real file upload in the first place, just a string that
happens to contain an @ symbol and a filename.

THE FIX:

curl -X POST https://api.example.com/upload -F "file=@photo.jpg"

Using -F instead of -d builds a genuine multipart/form-data body,
where @photo.jpg IS correctly interpreted as "read this file from disk
and include its actual contents as this field's value" — exactly the
distinction the chapter draws between -d and -F building fundamentally
different body formats, not interchangeable syntax for the same idea.