.docx MIME Type and Content-Type Header

About Word document files

A .docx file is a ZIP archive of XML files (Office Open XML). That long type name is the only one Office and browsers recognise; servers that don’t know it often send application/zip or application/octet-stream, and the file then downloads with a .zip extension in some browsers.

To tell .docx, .xlsx and .pptx apart without trusting the extension, open the ZIP and look for word/, xl/ or ppt/ folders.
Templates (.dotx) and macro-enabled documents (.docm) have their own types: …wordprocessingml.template and application/vnd.ms-word.document.macroEnabled.12.

File signature

DOCX files start with 50 4B 03 04 ("PK", a ZIP archive). The extension and the type a browser reports for an upload both come from the file name, so check these bytes on the server before trusting a file.

Serve .docx with the right Content-Type

Response header

Content-Type: application/vnd.openxmlformats-officedocument.wordprocessingml.document

nginx (a line in the types { } block of mime.types)

application/vnd.openxmlformats-officedocument.wordprocessingml.document docx;

Apache (.htaccess or server config)

AddType application/vnd.openxmlformats-officedocument.wordprocessingml.document .docx

Caddy

@docx path *.docx
header @docx Content-Type "application/vnd.openxmlformats-officedocument.wordprocessingml.document"

Check what a server sends

curl -sI https://example.com/file.docx | grep -i '^content-type'
# Content-Type: application/vnd.openxmlformats-officedocument.wordprocessingml.document

Set the type in code

Node.js (Express)

res.set('Content-Type', 'application/vnd.openxmlformats-officedocument.wordprocessingml.document');
// Or make the browser download it:
res.attachment('file.docx');

Python

import mimetypes

mimetypes.add_type('application/vnd.openxmlformats-officedocument.wordprocessingml.document', '.docx')  # if your system doesn't know it
mimetypes.guess_type('file.docx')[0]  # 'application/vnd.openxmlformats-officedocument.wordprocessingml.document'

mime.AddExtensionType(".docx", "application/vnd.openxmlformats-officedocument.wordprocessingml.document")
w.Header().Set("Content-Type", "application/vnd.openxmlformats-officedocument.wordprocessingml.document")

Accept .docx uploads

HTML

<input type="file" accept=".docx,application/vnd.openxmlformats-officedocument.wordprocessingml.document">

The accept attribute only filters the file picker; users can still choose another file. Listing the extension as well as the type matters, because operating systems don’t always report the type you expect.

Related types

.doc (application/msword): Word 97–2003 document
.xlsx (application/vnd.openxmlformats-officedocument.spreadsheetml.sheet): Excel workbook
.pptx (application/vnd.openxmlformats-officedocument.presentationml.presentation): PowerPoint presentation
.odt (application/vnd.oasis.opendocument.text): OpenDocument text

Sources

Spotted a mistake? Email hello@devtoolsmith.com and we’ll fix it.

Frequently asked questions

What is the MIME type for .docx files?

application/vnd.openxmlformats-officedocument.wordprocessingml.document. The full header is "Content-Type: application/vnd.openxmlformats-officedocument.wordprocessingml.document".

Why does my server send the wrong Content-Type for .docx?

Web servers pick the type from the file extension using a table (mime.types in nginx and Apache). If .docx is missing from that table, they send a default such as application/octet-stream or text/plain. Add the mapping as shown on this page, then check the response with "curl -sI".

How can I check that a file really is DOCX?

Look at the first bytes rather than the extension or the type the browser reports, which both come from the file name. DOCX files start with 50 4B 03 04 ("PK", a ZIP archive). The MIME type lookup on this site reads them for you when you drop a file on it.

DOCX MIME Type