Throw exception on passing MultipartFormDataContent in AddPinataFile

**Context**

Hi, thanks for your Pinata client first of all.
I needed to upload a directory with files. In `PinFileToIpfsAsync` I've passed `Action` in which I've called `AddPinataFile` only once and passed `MultipartFormDataContent` object in it. Since `MultipartFormDataContent` is a heir of `HttpContent` code worked correctly, but when I opened Pinata I saw, that I've uploaded a file without extension.
I was puzzled until I opened your source code and after that I've replaced one call of `AddPinataFile` passing `MultipartFormDataContent` with foreach and passing `ByteArrayContent`.
It is not intended to use `MultipartFormDataContent` in `AddPinataFile`, so maybe it should be prevented?


**Alternatives**

Throw `ArgumentException` when user passes `MultipartFormDataContent` object in `AddPinataFile`.

**If the feature request is approved, would you be willing to submit a PR?**

Yes