Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/340366

**Problem**

Golang `mime` package skips processing unsupported encodings (see
https://sourcegraph.com/github.com/golang/go@0fd0639e4c429e147d33bfc42654fcd651f4449f/-/blob/src/mime/mediatype.go?L247).

Because of that workhorse does not incercept the upload and skip
sanitization for filename.

**Solution**

Manually detect the charset of the `filename*` and reject the upload
when charset is unsupported.

Changelog: fixed