By default, maximum file size you can upload to Golf application server is about 25MB. You can change that by using "--maxupload" option in gg command line utility when you build your application:
gg -q --maxupload=100000000
In this case, maximum size of an uploaded file is about 100MB.
In order to know in your application (at run-time) what is this limit, use get-app:
get-app upload-size to up_size
pf-out "Upload size is %ld\n", up_size
For an example of uploading files, see file manager example.