feat: add support for legacy binary office formats

[ceberam]

Summary

Add support for legacy binary Office formats - .doc (Word 97–2004), .xls (Excel 97–2004), and .ppt (PowerPoint 97–2003) - by converting them to their modern Open XML equivalents via LibreOffice before delegating to the existing
MsWordDocumentBackend, MsExcelDocumentBackend, and MsPowerpointDocumentBackend parsers.

Resolves #1133

Motivation

Docling already supports .docx, .xlsx, and .pptx. Many real-world document collections still contain the older binary formats, which share the same logical structure. LibreOffice is already an optional dependency used elsewhere in Docling (EMF/WMF image rendering, chart rendering), so reusing it here keeps the dependency footprint unchanged.

Changes

New convert_to_modern_format utility

Shared helper that invokes LibreOffice headless to convert a legacy file (accepted as either a Path or a BytesIO stream) to a modern Open XML format and returns a BytesIO buffer.
Cleans up its temporary directory in all cases.

Extended backends

Each of the three existing backends gains support for its legacy counterpart:

Backend	New format
MsWordDocumentBackend	InputFormat.DOC (.doc, .dot)
MsExcelDocumentBackend	InputFormat.XLS (.xls, .xlt)
MsPowerpointDocumentBackend	InputFormat.PPT (.ppt, .pot, .pps)

When in_doc.format is a legacy format, __init__ calls convert_to_modern_format before the normal parsing path runs. No new backend classes are introduced.

Format registration

• InputFormat.DOC, InputFormat.XLS, InputFormat.PPT added to the InputFormat enum.
• Extension and MIME type mappings added to FormatToExtensions and FormatToMimeType in base_models.py (official MIME types: application/msword, application/vnd.ms-excel, application/vnd.ms-powerpoint).
• _mime_from_extension in document.py updated to handle the new extensions.
• _get_default_option in document_converter.py maps each legacy format to the same FormatOption as its modern counterpart.

Correct DocumentOrigin.mimetype

All three backends' convert() methods previously hardcoded the modern MIME type. They now use FormatToMimeType[self.input_format][0], so a .doc file produces mimetype="application/msword" in DocumentOrigin rather than the MIME type of the converted intermediate. All three legacy MIME types are already
accepted by DocumentOrigin's validator via Python's standard mimetypes module — no changes to docling-core are required.

Documentation

• docs/usage/supported_formats.md — new row for DOC, XLS, PPT in the supported input formats table.
• pyproject.toml — updated comment on the format-office extras block.

Limitations

• LibreOffice must be installed and on PATH (or at the standard macOS
  location). Conversion raises RuntimeError when it is not available; the
  end-to-end tests are automatically skipped.

Checklist:

• Documentation has been updated, if necessary.
• Examples have been added, if necessary.
• Tests have been added, if necessary.

[github-actions]

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[codecov]

Codecov Report

❌ Patch coverage is 22.50000% with 31 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
docling/backend/docx/drawingml/utils.py	11.11%	16 Missing ⚠️
docling/datamodel/document.py	0.00%	6 Missing ⚠️
docling/backend/msexcel_backend.py	25.00%	3 Missing ⚠️
docling/backend/mspowerpoint_backend.py	40.00%	3 Missing ⚠️
docling/backend/msword_backend.py	25.00%	3 Missing ⚠️

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