Invalid Content-Type Header Error

Match the Content-Type header to the actual format of your request body.

// JSON body
fetch("/api/data", {
  method: "POST",
  headers: { "Content-Type": "application/json" },
  body: JSON.stringify({ key: "value" }) // ✅ Correct
})

// Form data (let browser set Content-Type automatically)
const formData = new FormData()
formData.append("file", fileInput.files[0])
fetch("/api/upload", { method: "POST", body: formData }) // ❌ Don't set Content-Type manually for FormData

Even a small typo in Content-Type will cause it to be unrecognized.

// ❌ Common typos
"application/json"     // extra double quote
"application/json"     // missing slash
"applicaiton/json"     // misspelled
"Application/JSON"     // wrong case

// ✅ Correct
"application/json"
"application/x-www-form-urlencoded"
"multipart/form-data"

Check API documentation for the exact Content-Type the server expects.

// API expects
Content-Type: application/json

// But you sent
Content-Type: text/plain
{"key": "value"}

// Server cannot parse plain text as JSON → 400 or 415