Here is quote from
iCab Q&A:
Why is the file upload feature not working on file hosting sites like rapidshare and megaupload?
The iOS does not support file upload at all, so iCab has to take the file upload task out of the web page context and do it itself. This works fine as long as a web page lets the browser do its job when processing forms. iCab recreates the standard form processing when doing a file upload, so this is exactly what the browser would have done if file upload would be supported. But file hosters like rapidshare and megaupload do not want that the browser is doing its "job" and therefore these pages are overwriting the default form processing with their own form processing code. These pages disable iCab's own upload feature by overwriting it with their own upload code, and then they can only use the features which are supported by the iOS. And because the iOS does not support file uploads, you can't upload files on these web sites.
Also other sites rely on their own upload code instead of letting the browser do its job, like "yahoo mail" where you can not upload files for attachments.