Microsoft Excel XLS MIME Type

What does application/vnd.ms-excel mean?

application/vnd.ms-excel is the registered media type for Microsoft Excel spreadsheets. Microsoft registered it as a generic Excel identifier, independent of the application's version, subtype, or platform.¹

Today, the type is most closely associated with the legacy .xls workbook family. Excel 97 through Excel 2003 used the BIFF8 binary format by default. Earlier Excel releases also used .xls, but their Binary Interchange File Format, or BIFF, records are not necessarily identical.²³

The related .xlt extension identifies an Excel 97-2003 template. Microsoft describes both .xls workbooks and .xlt templates as binary Excel formats, while LibreOffice maps its corresponding import and export filters to application/vnd.ms-excel.³⁴

Why is 1996 listed as the registration date?

Microsoft submitted application/vnd.ms-excel for registration in February 1996 and followed up with IANA on March 25, 1996. The registration requires no parameters, allows an optional name parameter, and describes the object as a spreadsheet.¹

Excel and .xls files are older than the MIME label. Early Excel versions used BIFF2, BIFF3, and BIFF4 as single worksheet streams. Excel 5.0 introduced BIFF5 and the compound-file workbook structure in 1993. Excel 97 introduced BIFF8 in 1997, which remained the default through Excel 2003.²

Microsoft began publishing the binary workbook specification under its Open Specification Promise in 2007. The maintained [MS-XLS] specification documents the BIFF8 structures used by Excel 97, Excel 2000, Excel 2002, and Excel 2003.⁵

How can you identify an XLS file?

Most Excel 5.0 and later .xls workbooks begin with the eight-byte Compound File Binary signature:

D0 CF 11 E0 A1 B1 1A E1

Those bytes identify the container, not Excel. Legacy Word and PowerPoint files, Windows Installer packages, and other compound files can have the same header. A BIFF workbook also contains a Workbook or older Book stream with records describing workbook globals and individual sheets.²⁵⁶

This page therefore does not list the shared compound-file header as a definitive magic number. Reliable identification checks the filename, declared media type, compound-file directory, and BIFF records together.

Some very old .xls files make detection more complicated. Excel 2.0 through 4.0 workbooks can be plain BIFF streams rather than compound files, and a file saved for compatibility may contain more than one BIFF generation.² Renaming a CSV file or modern .xlsx package to .xls does not convert its contents.

How do you open an application/vnd.ms-excel file?

General-purpose browsers do not parse BIFF workbooks themselves. Use a spreadsheet application or a web service:

• Microsoft Excel opens Excel 5.0/95 and Excel 97-2003 .xls workbooks, as well as .xlt templates.³
• LibreOffice Calc imports and exports several legacy Excel generations under application/vnd.ms-excel.⁴
• Excel for the web can open .xls workbooks, but converts them to the newer .xlsx format for browser editing.⁷

Support in Poly

Poly recognizes application/vnd.ms-excel and .xls filenames as spreadsheets. When processing succeeds, you can:

• Browse the workbook with a first-sheet thumbnail and a connected spreadsheet preview.
• Search readable cell content and find visually related rendered sheets.
• Ask the Poly agent about text and images in the processed sheets.

The interactive preview requires a connection. Poly presents the workbook's readable sheet content, but it does not run VBA or Excel 4.0 macros, refresh external data connections, or behave like a live spreadsheet calculation environment. Password-protected, damaged, or incompatible workbooks may not produce a preview or content index.

Use a spreadsheet application when you need to edit cells, audit formulas, refresh data, or navigate a large model. Poly's preview and agent support are designed for finding and understanding readable workbook content.

Poly does not currently infer .xlt from a filename. Supply the correct media type when importing a legacy template if you want Poly to treat it as an Excel spreadsheet.

Should you convert XLS to XLSX, PDF, or CSV?

Choose a destination based on what must survive:

Microsoft has used XLSX as Excel's default workbook format since Office 2007, while retaining .xls for compatibility.²³ Excel for the web also converts .xls to .xlsx before editing.⁷

Conversion creates a new representation. Renaming budget.xls to budget.xlsx leaves the BIFF bytes unchanged. For an important workbook, preserve the original, create an editable modern copy, and export reviewed PDF or CSV copies when those simpler representations are useful.

Security and privacy considerations

Treat an unexpected XLS attachment as active-capable content. A legacy workbook can contain VBA, Excel 4.0 macro sheets, external links, embedded OLE objects, and data connections. Microsoft's current default macro setting disables macros with a notification, and Microsoft warns that enabling all macros can expose a computer to malicious code.⁸

External content can also create privacy and integrity risks. Microsoft blocks external workbook content by default because linked images and data connections can contact remote systems, disclose information, or access data with the recipient's credentials.⁹ Enable links or connections only when you trust both the workbook and its sources.

Workbooks may preserve author and company properties, comments, hidden sheets, hidden names, formulas, cached values, and links to local or network locations. A printable view does not reveal everything stored in the file. Inspect workbook properties, hidden content, links, and macros before publishing sensitive data.

Conversion is not sanitization. It can make a review copy easier to inspect, but a successful preview does not prove that the original workbook is safe.

XLS compared with XLSX

XLSX is usually the practical editable format for a new workbook. Keep XLS when a legacy workflow requires it, when a system specifically exports it, or when preserving the original file matters.