...
In order to pass a file as workflow variable, this is always handled in outbound-mode in PIPEFORCE. This means not the file itself is set as variable value but a reference to it. This is called a content reference which is a JSON contain all required information to load and show a file. See here: Content References (Files).
Whenver a file (content object) is passed as a variable to workflow.start, it will be automatically converted to a content reference JSON. This content reference is then passed to the workflow engine and any subsequent tasks and pipelines.