procedure Decode(
    const AIn: string; 
    const AStartPos: Integer = 1; 
    const ABytes: Integer = -1
); override;

Decode is an overridden procedure that implements line-oriented decoding for Base64-encoded values. 

TIdDecoderMIMELineByLine maintains an internal buffer for decoding operations that use Base64-encoded values that are not a multiple of 4-bytes. In other words, the FillChar may not have been appended to the end of the encoded values. Decode ensure that any remaining values in the internal buffer are included before added new values specified in AIn. 

Decode determines if the number of bytes in the input values is a multiple of 4 (for the 4-byte to 3-byte decoding operation). When it is not a multiple of 4, the unused bytes by are retained in an internal buffer for use in subsequent calls to the Decode method. 

Use Clear to discard any remaining bytes prior to starting the decoding operation. 

Decode calls the inherited method to perform Base64 decoding for the values specified. 

Call FinishDecoding to ensure that unused bytes in the internal buffer are padded with one or more values from the FillChar property and decoded.