For accounting periods ending after 31 March 2010, you must submit your tax computations and, with very few exceptions, the accounts that form part of your Company Tax Return, in inline eXtensible Business Reporting Language (iXBRL) format
You can only include one iXBRL Statutory Acccounts file as part of your Company Tax Return. If you're including iXBRL accounts as part of the online return, any additional accounts forming part of the return must be attached as PDF files. You must use a lowercase suffix (.pdf) with the file name or the upload will fail.
You can find detailed guidance on HM Revenue ands Customs Corporation Tax online filing requirement and using iXBRL on the Corporation Tax online filing and electronic payment webpage.