.json MIME type

Content-Type: application/json. It compresses well, so let the server apply gzip or Brotli.

application/json

JSON data. UTF-8 by definition, so the type takes no charset parameter.

Good to know

text/json and text/x-json are non-standard; some old clients send them, but servers should respond with application/json.
Express needs app.use(express.json()) to parse request bodies, and only parses requests whose Content-Type matches.
Formats built on JSON use the +json suffix: application/ld+json, application/problem+json, application/manifest+json.

The response header to send:

Content-Type: application/json

Most servers pick the type from the file extension. If yours sends the wrong one, map it explicitly:

nginx (inside the types block in mime.types)

application/json json;

Apache (.htaccess or server config)

AddType application/json .json

Express

res.set("Content-Type", "application/json");

Python

import mimetypes

mimetypes.add_type("application/json", ".json")

HTML file input

<input type="file" accept=".json,application/json">

See what a server sends for a file, and what type a local file really is by its contents:

curl -sI https://example.com/file.json | grep -i '^content-type:'
file --mime-type file.json

application/json.

Content-Type: application/json. It compresses well, so let the server apply gzip or Brotli.