The congressional committees probing the Iran- contra scandal expect to miss an Oct. 31 deadline for filing a final report and plan to ask for an extension of up to two weeks, a spokesman said. The spokesman for the select Senate panel said the committees planned in the next few days to request that the full House and Senate grant them an extension, probably until mid-November. The two committees technically will go out of business at the end of October if the chambers do not agree to an extension. Such an extension would be considered a formality and would likely be approved. The Senate aide said there was no major problem with the report and most of the writing would be complete by the end of the month.