What code do you have tried? Did you have an problems with it or any errormessages. You should tell us...
Otherwise i would suggest to USE the forumsearch on how to use PostFile. There are A LOT of examples you will find...
And you also should make sure the "serverside" CAN handle large uploads with a post-request...
For ex if the server is a normal webserver with php running and you want to post a file to it... PHP in this case may be limited to the size a post-request is allowed to have... Or the time running this php is limited.
Without seeing code and without knowing more informations about what exactly you have a problem with... It´s just guessing what we can do.