Unfortunately, the file upload APIs in Portals are actually part of portals itself. They're intended for hosting images and other assets for use in Portals applications.
What are you wanting to do with these files? The OneP does have some ability to host files, but only for individual client models. That is to say, you can upload a file in the provisioning system that require an activated client of that client model to authenticate to be able to download the file.
Also, if the file is small and only for use by a single client you could write it to a string dataport, escaping any non-utf8 or base64 encoding for binary files. The limitation here is that a string dataport can only contain 64k bytes. (We have had customers that split their files and write them to several timestamps in a row to get around this, but I don't suggest doing this as it's complicated to make it reliable.)