[BUG] DownloadHttpFile Activity - Using storage : workflow variable will result in a server error

## Description
Using a workflow variable storage for a DownloadHttpFile activity will result in an error.

## Steps to Reproduce
1) create a simple download file workflow like so:
2) in the download file: output : result -> set the Result to a variable.
3) have the variable declared like so:
4) call the http endpoint

4. **Reproduction Rate**: Every time.

5. **Video/Screenshots**:

## Expected Behavior
No error should be thrown, maybe not allow variables of that type to be used for the activity.

## Actual Behavior
Error is thrown.

## Environment
Client version: 3.2.0-rc2
Server version: 3.2.0-rc2

## Additional Context
If you se the variable storage type to memory, the error disappears.