...
Content Upload methods use a separate SOAP endpoint from all other Web API methods. To upload a file, the client must first call StartFileUpload(), then call AppendChunk() one one or more times, then call CompleteFileUpload(). It is also possible to upload multiple associated webpage asset files using a session token provided by the StartWebPageUploadSession() method method. The workflow for uploading webpage assets is outlined in the following section.
The Content Upload service allows uploading files using multiple threads in parallel. To use multi-threading, the client must first call StartFileUpload() and and obtain an upload token. The client then uploads chunks in multiple threads (using AppendChunk() with with different parameters), specifying the partNumber and offset for each thread along with the same upload token. After the chunks are uploaded, the client must call CompleteFileUpload(). The client may also upload several different files while at the same time specifying different upload tokens.
...