mime/multipart: add helper to build content-disposition header contents

[malta895]

This PR adds an helper FileContentDisposition that builds multipart
Content-Disposition header contents with field name and file name,
escaping quotes and escape characters.

The function is then called in the related helper CreateFormFile.

The new function allows users to add other custom MIMEHeaders,
without having to rewrite the char escaping logic of field name and
file name, which is provided by the new helper.

Fixes #46771

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