MIME Types Extension Checker
Validate MIME types and file extensions with mismatch checks and bulk lookup.
MIME Types Extension Checker
Look up MIME types by extension (or extensions by MIME) and flag mismatches.
About MIME Types Extension Checker
MIME Types Extension Checker for File Extension and Content-Type Validation
When a download renders as gibberish, an image refuses to preview, or an API rejects an upload, the root cause is often simple: the file extension and the MIME type do not agree. This MIME Types Extension Checker helps you check the relationship between a filename extension (like .png) and a media type (like image/png) so you can fix headers, naming, and validation rules with confidence.
You can run two kinds of checks. First, paste filenames or extensions to look up the MIME types that are commonly expected for those formats. Second, paste MIME types to see the extensions that are typically used when saving files on disk. An optional “declared MIME type” field lets you compare what a client or server claims against what the extension usually implies.
The output is designed to be practical: one line in, one result block out, plus a short summary of how many entries were recognized, how many were unknown, and how many would be considered mismatches. Copy it into a bug report, attach it to an internal ticket, or save it as a text file for audit notes.
How It Works
MIME types (also called media types) are standardized identifiers like text/html, application/json, or image/webp. They appear most often in HTTP headers such as Content-Type, but they also show up in email attachments, multipart form uploads, and messaging systems that need to label binary payloads. The MIME type tells a consumer what to do: render it, download it, parse it as JSON, or treat it as an opaque stream.
File extensions are a human-friendly convention that helps operating systems and users guess what a file is. Extensions matter for search, UX, and default app associations, but they are not authoritative. A malicious or mistaken file can be named invoice.pdf while actually being a script, a ZIP archive, or random bytes. That is why robust systems use more than one signal.
This checker focuses on the “mapping layer”: given an extension, what MIME types are typically associated with it across common server configurations and libraries? And given a MIME type, what extensions are most often used when saving files? The tool uses a curated list of widely used mappings for everyday web formats (HTML, CSS, JS), documents (PDF, Office files), data (JSON, CSV), media (audio/video), and archives (ZIP, TAR, GZIP). It then builds a reverse index so the same dataset supports both directions.
If you provide a declared MIME type (for example, what a browser sent in a multipart upload, or what a server returned in a response header), the checker compares it to the expected MIME list for each extension. A match means the declared value is among the known options. A mismatch means the declared value is not typical for that extension and is worth investigating.
Quick steps
- Choose a mode: check by filename/extension or look up by MIME type.
- Paste input lines: one filename, extension, or MIME type per line.
- Optional validation: add a declared MIME type to detect mismatches quickly.
- Review the report: copy or download the output for sharing and record keeping.
What the tool does not do
- No file sniffing: it does not inspect file bytes or magic numbers. It is a mapping and validation helper.
- No environment assumptions: it does not depend on your server’s installed MIME map, which can vary by OS and configuration.
- No risky guesses: when a format is ambiguous, it reports multiple common MIME types instead of pretending there is only one.
Key Features
Extension → MIME lookup
Enter photo.jpg, .svg, or json and get the expected MIME type(s). This is useful for building allowlists, configuring object storage metadata, or debugging why a browser refuses to render a resource inline. For example, if your CSS file is served with text/plain, some browsers will not apply it as a stylesheet, and strict security headers may block it entirely.
MIME → extensions lookup
Paste MIME types such as application/json, text/csv, or video/mp4 and retrieve common extensions. This helps when an API returns a MIME type and you need to name a downloaded file correctly, when your application generates files dynamically, or when you are creating a support playbook for customers.
Mismatch detection with declared MIME
If a client claims Content-Type: application/pdf but the filename ends with .jpg, you want to know. The checker highlights match versus mismatch so you can fix headers, rename files, or harden upload validation. Mismatches also explain “why did my download open in the wrong app?” issues: the extension suggests one thing, while the header suggests another.
Batch-friendly, line-preserving output
Work in bulk: paste a list from logs, storage listings, or support tickets. The output keeps each original line, the normalized value the tool extracted, and the best-known mapping. This makes it easy to reconcile results with the original source, even when filenames include multiple dots, query-like suffixes, or mixed casing.
Config-ready reporting
The result is plain text and structured for copy/paste into documentation. Use it to create a concise allowlist for upload endpoints, to validate the values you set in Content-Type headers, or to check the mappings you plan to add to a web server, CDN, or object storage metadata rule.
Copy and download output
One click copies the full report to the clipboard, and another downloads it as a plain text file. This is ideal for audits, bug reports, and change requests where you need an artifact. The downloaded file is intentionally simple so it can be attached to issues, pasted into chat, or stored alongside release notes.
Use Cases
- Upload validation: ensure user uploads match allowed types and reduce spoofing risk in forms and APIs.
- API troubleshooting: verify that Content-Type headers align with the files being sent, especially in multipart uploads.
- CDN/object storage metadata: set correct MIME types so browsers cache and render assets properly (CSS, JS, fonts, images).
- Download naming: choose a sensible file extension when you only know the MIME type from an upstream service.
- Email/attachment handling: map attachment names to expected media types for scanners, previews, and policy checks.
- Documentation: generate accurate lists of supported formats for product docs and onboarding guides.
- Security reviews: build allowlists/denylists that consider both extension and MIME, not just one field.
In short, anytime you have a filename on one side and a Content-Type on the other, this checker helps you align them—preventing broken previews, incorrect downloads, and avoidable security gaps. It is also helpful for teams that maintain multiple environments (local, staging, production) where MIME maps can drift over time, producing hard-to-reproduce behavior.
Common symptoms of mismatched types include: browsers downloading files instead of displaying them; fonts failing to load due to incorrect types; JSON endpoints being treated as plain text; CORS or security middleware blocking responses when types do not match expectations; and file previews in SaaS apps showing “unsupported format” even though the underlying data is fine. A quick mapping check can save hours of trial and error.
Optimization Tips
Validate with more than one signal
Extensions and MIME types are helpful hints, but they can be wrong or malicious. For uploads, pair this tool’s mapping with server-side “magic number” detection (file signature sniffing), size limits, and content scanning. Treat client-provided MIME types as untrusted input. If you handle images, consider verifying decodeability rather than trusting headers or names.
Prefer canonical MIME types
Some formats have historical variants (for example, JavaScript can appear as application/javascript or text/javascript; XML can show up as application/xml or text/xml). Pick a canonical value for storage and HTTP responses, then accept known alternates when validating incoming requests. Consistency reduces surprises in browsers, proxies, and downstream services.
Use “nosniff” intentionally
Modern security best practices often include X-Content-Type-Options: nosniff, which tells browsers not to guess a type. With nosniff enabled, incorrect MIME types break rendering more often—especially for scripts and stylesheets. That is a feature, not a bug: it forces correct configuration. Use this tool to verify your asset types before enabling stricter security headers.
Keep your allowlists focused
A broad allowlist increases risk. Use your product requirements to choose the smallest set of extensions and MIME types that still meets user needs. If you only support images, do not allow archives or executables—even if they can technically be served. When you do allow archives, be explicit about which ones and consider scanning contents server-side.
Document the decision
When you pick canonical types and supported extensions, write the decision down. Add the mapping to your codebase, server config, and runbooks. The output from this checker can serve as a quick, human-readable appendix that explains what you accept and why, which reduces churn when new team members join.
FAQ
A MIME type (media type) is a standardized label like image/png or application/pdf. Web servers send it in the Content-Type header, browsers use it to decide how to handle a response, and APIs use it to interpret request bodies and attachments. It also appears in multipart form parts, email attachments, and many storage systems that preserve metadata.
Correct MIME types improve user experience (inline display vs forced download), interoperability (parsers know what to expect), and security (browsers and gateways can apply stricter handling rules). Incorrect types lead to “it works on my machine” bugs when different clients sniff content differently.
No. Extensions are easy to change, and some systems generate misleading names. Use extensions for user experience and quick filtering, but rely on server-side inspection (file signature sniffing) when security matters. For critical workflows, validate that the file can actually be parsed or decoded as the claimed type.
That said, extensions still matter: they influence how operating systems open files, how users understand attachments, and how some servers apply default MIME maps. Treat them as helpful metadata—just not a source of truth.
Different ecosystems and older standards sometimes use alternate labels for the same format. Servers, frameworks, and browsers may prefer different canonical values, and legacy configurations can persist for years. When the tool shows multiple options, choose one canonical MIME type for your system and accept reasonable alternates during validation.
In practice, you should standardize on a single value for storage and outgoing responses. For incoming requests, accept a short list of safe alternates and reject anything unexpected, especially for executable or script-capable types.
Compound extensions describe layered formats: a TAR archive compressed with GZIP. Many systems treat the “last” extension as the primary one (.gz), while others preserve the full chain (.tar.gz) for clarity. When you validate uploads, decide whether you allow archives at all, and if you do, consider validating both layers to avoid “archive bombs” or hidden executable content.
If your users commonly upload compressed archives, be explicit in your UI and docs about what you accept, how you unpack, and what you scan. The mapping check is the first step; content inspection is the second.
It complements configuration rather than replacing it. Use this checker to decide what your system should send and accept, then implement those decisions in your server, framework, CDN, and upload validation rules. The report is especially useful for debugging when a misconfigured map causes a file to download instead of render, or when a proxy rewrites headers unexpectedly.
Think of it as a planning and debugging aid: it gives you a consistent reference point that is independent of any single machine’s MIME database. Once you standardize your mapping, you can enforce it in code and configuration.
Why Choose This Tool
You do not need a sprawling reference encyclopedia to solve most real-world MIME problems—you need fast, consistent answers for the formats you actually handle. This MIME Types Extension Checker prioritizes common web, document, archive, audio, and video formats so you can ship correct headers, safer upload rules, and cleaner integrations without digging through multiple sources every time.
Because it supports batch input, mismatch checking, and export-friendly output, it fits naturally into developer workflows: debugging a failed API call, reviewing an upload allowlist, documenting supported formats, or responding to a security question. Use it as your lightweight “sanity check” whenever extensions and MIME types disagree, then follow up with file inspection and scanning when the stakes are higher.