Office Open XML: Invalid Files Return Success with Error Message Instead of Exception

When converting invalid DOCX, XLSX, or PPTX files, MarkItDown returns a successful result with the string "This is not a valid Office Open XML file." in text_content, rather than raising an exception. This behavior makes it difficult to distinguish between a successful conversion and a file format error.

Expected Behavior:Invalid or corrupted Office Open XML files should cause the converter to raise a specific exception (e.g., FileConversionException), not return a result with an error message in the content.

Actual Behavior: The conversion completes "successfully" and returns a DocumentConverterResult with text_content set to "This is not a valid Office Open XML file.".

Steps to Reproduce:

Attempt to convert a non-OOXML or corrupted DOCX/XLSX/PPTX file.
Observe that no exception is raised; instead, the result contains the error string.
Suggested Fix: After conversion, check if text_content matches the error string and raise an exception if so. Alternatively, ensure the underlying libraries raise on invalid files and propagate the exception.

Additional Context: This behavior appears consistent across all Office Open XML converters, possibly due to a shared dependency or error-handling pattern.

Workaround:

    result = markitdown.convert_stream(stream=stream, file_extension=ext),
    if result.text_content.strip() == "This is not a valid Office Open XML file.":
        raise ...
    return result.text_content

Start at the markitdown.convert_stream entry point and reproduce the behavior with an invalid or corrupted DOCX, XLSX, or PPTX file. Trace the shared Office Open XML conversion path and confirm that invalid input raises a specific exception instead of returning the error string in text_content; verify the result for all three formats.