MIME Types and HTTP Status Codes - Quick Browser Reference

This guide is a quick reference workflow, not RFC memorization. Use MIME Type Lookup and HTTP Status Code Reference when you are configuring uploads, reading CDN errors, or explaining incidents without Stack Overflow roulette.

MIME: extension lies, Content-Type contracts

Browsers and APIs trust Content-Type (and sniffing rules) more than filename extensions. Wrong type causes:

• Download instead of render (Content-Disposition + application/octet-stream)
• XSS when text/html is served for user uploads
• Broken charset (text/plain without charset=utf-8)

Common fixes:

Symptom	Check
.webp not displaying	Type image/webp on object storage
JSON parsed as text	application/json; charset=utf-8
.wasm fails to instantiate	application/wasm
SVG uploaded by users	image/svg+xml - sanitize; SVG is XML

MIME Type Lookup maps extension ↔ type for quick sanity. Official registry is huge; tools curate common dev types.

HTTP status: who is responsible?

Rough ladder:

• 1xx - informational (rare in app dev)
• 2xx - request reached server logic successfully
• 3xx - redirect; watch Location and cache behavior
• 4xx - client fault (bad auth, validation, not found)
• 5xx - server or gateway fault

HTTP Status Code Reference for precise phrases.

Incidents in plain English

401 Unauthorized - often missing/invalid auth (name is historical). Check Authorization header, cookie domain, clock skew on JWT exp - decode locally per JWT generate vs decode.

403 Forbidden - authenticated but not allowed. RBAC, WAF, geo block.

404 Not Found - route or object missing. Distinguish app 404 vs CDN 404.

408 / 504 - timeout somewhere (client, load balancer, upstream). Trace IDs beat guessing.

429 Too Many Requests - rate limit; respect Retry-After.

502 / 503 / 504 - gateway or upstream sick; not fixed by MIME changes.

Combine tools on one log line

Example log snippet (redacted):

GET /api/v1/report?token=REDACTED HTTP/1.1" 415
User-Agent: Mozilla/5.0 (... Chrome/141 ...)

Steps:

1. 415 → HTTP Status Code Reference - Unsupported Media Type: body Content-Type rejected.
2. Paste URL into URL Parser - confirm path and query keys; remove secrets from query in fix, not just in paste.
3. Paste UA into User Agent Parser - reproduce on matching browser.
4. If upload involved, verify file type in MIME Type Lookup.

Header deep dive: How to read HTTP headers.

Security cluster context

These references are read-only knowledge - they do not verify TLS certs or scan for CVEs. Pair with Best free security and encoding tools for encoding, JWT, and hash tasks.

JWT responses often wrap tokens in JSON (application/json) while attachments use discrete types - do not confuse token string MIME with resource MIME.

CDN and S3 checklist

• Set Content-Type at upload time
• Set Cache-Control separately - type does not imply caching
• Content-Encoding: gzip is not a MIME type swap
• application/octet-stream as lazy default breaks preview features

What lookup tools will not do

• Fix misconfigured nginx types {} blocks
• Tell you why your API chose 418
• Replace curl -v or browser Network panel

They shorten the gap between error code + filename and next hypothesis.

Keep MIME and status references one click away, parse URLs and UAs before you blame the database, redact query tokens, and read headers when 415 or 406 appears - the body type is usually the story.