Usually networks will not allow you to attach larger files to your MMS. Also the files you attach in your MMS should be popular media type files. The only solution you have is to share a larger file or a network not supported file type with a Sri Lankan mobile user, is that using a file sharing service optimized for mobile devices.
Use this link, compose your MMS and attach your file. This website allows you to attach up to 10MB per MMS. If the file is too larger to send via MMS, or if the file is not supported by the target network, then this website is using a mobile optimized file sharing method. You can send almost any file to a mobile user in Sri Lanka using this website. (Currently supported only with Dialog and Mobitel)
Recipient must have activated MMS on their mobile to receive the file successfully.