How are you attaching a file to your workflow? When starting a workflow, you are only allowed to enter values into the workflow's data fields. The only data type allowed for data fields are booleans (True/False), strings (text), number (integers), decimals, and date/time. Any thing other than that is not allowed.

Unless you convert your file into a base64 encoded string, and then format it into the following XML format:

<file>

  <name>filename</name>

  <content>...</content>

</file>

The above format is used by SmartObject to store files into SQL tables. But even so, I have never encounter anyone who attempt to pass a file into workflow. Usually its done through SmartObjects, by first uploading the file into a SQL table, then pass the identifier for the newly inserted record into the workflow. After which the workflow will call the SmartObject to retrieve the file.

It will be clearer if you can further elaborate how you pass the file into the workflow, and what you are trying to achive with passing the file into the workflow.