I doubt you can send files using HTTP. You can create a request using the HTTP plugin as I answered one of your previous question - Data Fusion - Issue with http post plugin.
In HTTP Callback you can find information:
This action can be used when you want to perform an HTTP request at the end of a pipeline run.
Using this plugin, you can send a request or text, not a file.
If you would like to send a file, you should use File
like it's mentioned in this thread: Some questions about google Data fusion
Regarding Error 500
The HyperText Transfer Protocol (HTTP) 500 Internal Server Error server error response code indicates that the server encountered an unexpected condition that prevented it from fulfilling the request.
It means that there is an issue with your endpoint. It might be a firewall or HTTP server issue. Too many possibilities of what could be wrong with current details.
In addition, you can find some additional information in doc Ingest CSV data to BigQuery using Cloud Data Fusion - Batch ingestion