I created a MS trouble ticket for this issue and we were able to identify the problem.
In the custom component code’s Execute method I was grabbing the <inputmsg>.MsgPart.Data property and checking for length etc. The file adatper implements the Data property as both a getter and setter. The http adapter on the other hand only implements the setter. Hence the error message. To get the data you need to use the call the GetOriginalDataStream method.
Hope this helps anyone else who runs into this inconsistency.
Geordie