7.1.8-beta.2

Pre-release for #216 — tracks PR #218.

Adds to 7.1.8-beta.1: Microsoft.AspNetCore.OpenApi now emits encoding.contentType for the file part (the backend Scalar uses), so the allowed media types are machine-readable in the document, not only in the description:

multipart/form-data:
  schema: { $ref: "#/components/schemas/UploadProductImageRequest" }   # net10
  encoding:
    File:
      contentType: "image/jpeg, image/png"

Works on net9 (inline form schema) and net10 (resolves the whole-body $ref component to find the part name). Backend matrix is now content-type+size on all three backends, both as description and encoding.contentType.

Pre-release built from branch feature/issue-216-media-types (not yet merged to master).