Please see topic Uploading large (and binary) files to Worklight adapter.
Basically, Worklight does not have the equivalent to an HTTP POST mechanism that allows you to transfer arbitrarily large chunked data. For large files of unknown sizes (photos, video, audio) you'll need to upload the file to the server outside the Worklight adapter framework. For example you could simply post it to a web server you have configured. In my case (in the above referenced answer) I needed to create an entire client-server mechanism to negotiate a port and key, start listening on that port, then accept requests and ensure the posting client passes the key as authorization to transfer the secure data.
Hopefully IBM will provide a formal service for this in a future release.