OOXML spreadsheets and presentations are detected as DOCX

[Vladipz]

MicrosoftOffice365Document identifies every OOXML ZIP container as a DOCX file.

Its magic-byte signatures are ZIP local-header variants and are not specific to Word documents. Therefore, .xlsx and .pptx files are also reported as:

• extension: docx
• MIME type: application/vnd.openxmlformats-officedocument.wordprocessingml.document

Reproduction

using FileTypeChecker;

using var stream = File.OpenRead("workbook.xlsx");

var result = FileTypeValidator.TryGetFileType(stream);

Console.WriteLine(result.Type.Extension); // docx
Console.WriteLine(result.Type.MimeType);  // application/vnd.openxmlformats-officedocument.wordprocessingml.document
The same happens for a .pptx file.
Expected behavior
- .docx should resolve to application/vnd.openxmlformats-officedocument.wordprocessingml.document
- .xlsx should resolve to application/vnd.openxmlformats-officedocument.spreadsheetml.sheet
- .pptx should resolve to application/vnd.openxmlformats-officedocument.presentationml.presentation
Suggested direction
OOXML files are ZIP containers, so magic bytes alone cannot distinguish the subtype. The detector needs to inspect package entries:
- word/document.xml for DOCX
- xl/workbook.xml for XLSX
- ppt/presentation.xml for PPTX
Legacy Compound File Binary formats have a similar ambiguity: .doc, .xls, and .ppt share the same container signature and should not all be reported as doc.