I managed to solve this with the help from Samuel at K2.

He advised that SmartObjects handle files with an XML structure containing the filename and the content as a Base64 string.
I then changed my smartobject property to be of the "File" type and then changed my web service to accept this property as string.
In my web service method I then converted it to XML and then converted the content from Base64 to a byte array and saved it to the back-end system.

Using this work-around I was able to submit files using this smartobject.

I hope this will help someone else who is struggling with the same thing.

Hi, Johan.

First of all thank you for posting here.

Сould you post some example your solution? I would like to reproduce the same web service.
Did you use this with smartforms?  

Hi,

I'm having the same issue as the OP.  I am trying to call an external web service that I can't modify and it requires a file passed in as a byte array.

There doesn't seem to be a resolution to this on this thread.  Has anyone found a way around this?

Regards

Phil

Sorry, I meant to post on this thread:

http://community.k2.com/t5/K2-blackpearl/How-to-send-an-array-of-byte-to-a-web-service-thru-a-smart/td-p/66732

Try something like below:

  private void GetFileFromXML(out bytee] fileContent, out string fileName, string attachment)
        {
            fileContent = null;
            fileName = null;

            var xDocument = XDocument.Parse(attachment);

            var file = (from x in xDocument.Descendants("file")
                         select new
                         {
                             FileName = x.Element("name").Value,
                             FileContent = x.Element("content").Value
                         }).FirstOrDefault();

            fileName = file.FileName;
            fileContent = Convert.FromBase64String(file.FileContent.ToString());
        }