Question 1

How does the tool know the real file type?

Accepted Answer

Most formats start with a fixed byte signature ("magic bytes"): PNG files begin with 89 50 4E 47, PDFs with %PDF, ZIP archives with PK. The tool compares the file's leading bytes against a database of these signatures, which is far more reliable than trusting the extension.

Question 2

How are large files handled?

Accepted Answer

Only the first 64 KB is read using File.slice(), which is lazy — the rest of the file is never touched. Inspecting a multi-gigabyte video therefore takes the same instant as a small image, and memory usage stays constant.

Question 3

Why does an extension mismatch matter?

Accepted Answer

A file whose content disagrees with its extension was usually renamed — sometimes innocently, sometimes to disguise an executable as a document. Checking the magic bytes reveals what a program will actually do when it opens the file.

Question 4

Why are some files reported as unknown or just "ZIP archive"?

Accepted Answer

Formats without a distinctive header (or proprietary ones not in the signature list) cannot be identified from bytes alone. ZIP-based formats like DOCX, APK, and EPUB are recognized when their internal entries appear early in the archive; otherwise the generic ZIP result with candidate extensions is shown.

Question 5

Is my file uploaded for analysis?

Accepted Answer

No. The file never leaves your device — the few leading bytes used for detection are read directly in your browser, nothing is uploaded or tracked, and the tool works offline once the page has loaded.

File Type Inspector

FAQ