# File extension vs file type > A file extension is a label after the last dot. A file type is what the bytes inside actually represent. Most of the time they agree; when they do not, the bytes are the truth. Source: ### Key facts - **Extension** The trailing label after the last dot in a filename. Trivial to change. A hint, not a contract. - **File type** The format identified by the bytes inside the file. Detected by reading the first few bytes (the magic-byte signature). - **When they diverge** When someone renames the file, when an extension was never set, or when the file is a polyglot (valid as more than one format at once). - **How we report it** Four verdicts: MATCH, MISMATCH, AMBIGUOUS, UNKNOWN. Each is a deterministic answer about format identity. ### Frequently asked questions #### Why does a DOCX file have the same magic bytes as a ZIP? Because DOCX is a ZIP archive. Microsoft Office documents (DOCX, XLSX, PPTX) are all ZIP containers carrying XML and embedded media inside. The magic-byte check sees the ZIP envelope first; the OOXML manifest inside the archive distinguishes the document type. #### Can two different formats share the same extension? Yes. `.dat`, `.bin`, `.tmp`, and many vendor-specific extensions are reused freely. The bytes are still the source of truth; the extension is just an unreliable hint. #### Is changing the extension ever the right move? Sometimes. If a file has the bytes of a JPEG but no extension, naming it `.jpg` is correct. The error is the other direction: changing the extension to disguise what the file is. #### What if my operating system disagrees with your verdict? Modern operating systems read the magic bytes too, but they cache file-type associations differently. Our checker reports what the bytes actually say, regardless of any cached association. If the two disagree, trust the bytes. > Drop the file into our File Type Checker. Four verdicts cover every case: MATCH, MISMATCH, AMBIGUOUS, UNKNOWN.