I think that this should be written the other way around to remove indentations and branching. For instance, I would write something like:

    if not arguments:
        raise InvalidRequestBodyFileError(
            "Please provide the file path to the request body."
        )

    body_file = Path(arguments.pop())
    if not body_file.exists():
        raise InvalidRequestBodyFileError(
            f"Request body file located at '{body_file.absolute()}' does "
            "not exist!"
         )
    if not body_file.is_file():
        raise InvalidRequestBodyFileError(
            f"Request body file located at '{body_file.absolute()}' is not "
            "a file!"
         )

    with body_file.open("r") as raw_body_contents:
        return json.load(raw_bady_contents)

That way, you can also remove the Union[X, NoReturn] return type and just specify the return type (because clearly that method will always return a dictionary or raise an exception).

For the json library, just to keep it consistent with the rest of the codebase, I would prefer if you imported just json and then used the json.load method ❤️

I think that if arguments: is a bit more elegant on this situation

I would loose the + operator (as the parentheses will concatenate the strings) and leave the space on the upper line, for consistency's sake

I think that this whole piece of code could be decoupled and implemented as a method that receives the output of calling raw_endpoint.get("body") and the remaining_arguments and returns the corresponding body. That way, this new function could operate all the conditionals and all that stuff then execute the load_request_body method directly with the first argument (if it exists)

I think that this method could explicitly say file, so that it is obvious that it retrieves the contents of a file. For example, it could be named load_request_body_from_file. Also, I think that it should receive only the first element of the arguments list (pre-processing that information outside this method). I don't know what should happen if arguments were to be empty, maybe you should pass None or directly raise the error from outside the method. I leave that to your judgement. Lastly, the return type should be only Dict[str, Any]. On another comment I left I explained how to stop mypy from whining about the return type by refactoring the method's code

I would loose the + operator (as the parentheses will concatenate the strings) and leave the space on the upper line, for consistency's sake