At this point, XlsFormatProvider doesn't support images and they are not imported into Workbook while reading an .xls file.

XlsFormatProvider: InvalidOperationException is thrown when importing a document with an unsupported BIFF record.

Workaround: Re-save the file with Excel before importing it.

At this point XLS import of SpreadProcessing does not have support for notes/comments.

Add support for superscript and subscript font effects for cell formatting. Currently, this is not supported by the model and is omitted on import.

XlsFormatProvider: Add support for hidden rows.

Add support for strikethrough font effect for cell formatting. Currently this is not supported by the model and is omitted on import.

Opening a file where there is a format string with modificator [Color11] causes an ArgumentException "Invalid modificator" This prevents the file from opening.

The XlsxPartNames class expects a strict name for reading the comments parts:

public const string CommentsPartName = @"/xl/comments{0}.xml";

However, some third-party tools may change the name to GemComments.xml for example. 

MS Excel recognizes this naming for the comments part. It would be nice to provide such functionality in RadSpreadProcessing.

RadSpreadProcessing expects an integer, instead of boolean. However, according to the specification, it should be boolean: