MIME Type Checker

The MIME Type Checker on Callculation lets you upload and analyze multiple files simultaneously to verify their true content type. It inspects each file’s binary header (magic bytes) and metadata to detect its actual MIME type — not just what the browser or extension claims.

The tool also highlights any inconsistencies between the browser’s file.type declaration and the real file signature (Detected MIME), helping developers and security researchers quickly identify mislabeled or spoofed files.

When you select multiple files, each is processed locally in your browser. The tool reads the first few kilobytes of every file, extracts its magic bytes, and compares them against a built-in MIME signature database using libraries like file-type and jschardet. All operations run securely on your device without uploading any data.

Every file the browser provides includes a MIME claim (via file.type), but this claim often depends on file extension and can be unreliable. The tool compares this claim to the true MIME derived from the file’s actual bytes, flagging any mismatches so you can verify accuracy.

When validating uploads, APIs, or data pipelines, it’s crucial to confirm that all files are of the expected format. Batch MIME checking saves time and helps ensure content integrity across large sets of files, especially during audits, penetration testing, or QA reviews.