Determine if we should re-instate proxying of MultipartFile method params

[onobc]

In #3258 we removed the ability to proxy (for projection) method params of type MultipartFile. The purpose of this issue is to dig in deeper on that decision and decide if we should support this.

• understand the current root cause of the breakage of the proxy handler supporting this
• summarize findings and decide on whether to support this
• add support back in (or not) depending on outcome from above