Content-Type Generator (MIME Header Builder)
Build Content-Type values with optional charset and quoted parameters
Choose type/subtype for the actual content. Parameters are optional; multipart needs a boundary matching its body.
The full guide also includes pitfalls, worked examples, snippets, FAQs, and related tools for checking results or troubleshooting.
About this tool
Choose a media type and subtype, then add parameters for the representation you are sending. The generator checks token syntax, quotes Charset and Boundary values when needed, parses semicolon-separated Extra params, and rejects duplicate names. Multipart requires a usable boundary. It displays both the field value and a complete Content-Type: line; the copy button copies the value. It does not generate a body, discover the content format, register a MIME type, or send a request.
Failure Clinic (Common Pitfalls)
Setting a boundary does not build a multipart body
Cause: The tool formats a declaration only; it does not serialize fields, files, or delimiter lines.
Fix: When sending browser FormData with fetch, let the browser set Content-Type and the matching boundary. For a manually serialized body, enter the boundary used by that exact body.
Adding charset does not re-encode content
Cause: A charset parameter describes bytes already produced elsewhere. application/json has no defined charset parameter.
Fix: Use application/json for JSON and leave Charset blank. For text/html, choose the actual encoding rather than changing only the declaration.
Scenario Recipes
Declare UTF-8 HTML
Goal: Build a header for HTML that is already encoded as UTF-8.
- Select text and enter html as the subtype.
- Enter utf-8 in Charset and leave Boundary and Extra params empty.
- Generate text/html; charset=utf-8 and use the value in the receiving application’s Content-Type field.
Result: The page also displays Content-Type: text/html; charset=utf-8 for contexts that require a complete header line.
Suggested Workflow
API Debugging Checklist: From Broken Response to Root Cause
A practical debugging flow for API payload errors, auth failures, and malformed responses.
Why 415 Unsupported Media Type Happens (and How to Fix It Fast)
Compare form and JSON requests for a JSON endpoint, verify the response, and distinguish browser FormData boundaries from hand-built multipart bodies.
Frequently Asked Questions
How do I enter custom parameters?
Use name=value pairs separated by semicolons. Quote an extra parameter value containing a space or semicolon, such as note="a;b". Do not repeat Charset or Boundary in Extra params when you already filled its dedicated field.
Does a generated value prove compatibility?
No. Syntax checks cannot establish that an endpoint accepts the type or that the body matches the declared charset or boundary. Check the receiving API and the actual body.
Does the tool keep a draft?
The form is saved in local storage in this browser and restored on return. Clear removes the saved draft. Header generation itself does not upload the form.
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