fix #2240 HTTP 400 error with vertx-web-openapi backend and Swagger UI

[bfreuden]

Motivation:

According to a discussion in the OpenAPI specification github repo (discussion including Mike Ralphson, a major OpenAPI contributor), it seems that:

1. */* is an acceptable value
2. application/octet-stream is not equivalent to */*:

OAI/OpenAPI-Specification#1692 (comment)

But it is true that, when omitted in the spec, the application/octet-stream mime type is implied (and this precisely what Vert.x is doing):

https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.0.1.md#special-considerations-for-multipart-content

Supporting the */* content type will allow writing upload forms that will accept any type of file sent by Swagger UI.

[pk-work]

I will close this PR because it looks abandoned and is related to a not longer maintained component.

In case I am wrong with my assessment, feel free to reopen the PR.