AssetManager: Configurable upload name suffix for multi file uploads

[merlinschumacher]

This PR adds the option multiUploadSuffix to the AssetManagerConfig. This makes it possible to replace the currently hard-coded value [] with a custom one or just an empty string.

To ensure the change causes no breakage for existing setups, the value of multiUploadSuffix defaults to []. Developers who would like to have another or no suffix at all can change it to their needs.

A related issue was raised already in #693, but closed with the argument, that the name property of a Content-Disposition line represents an array. I know the brackets are common practice in PHP, where they are processed as an indicator for an array of uploaded files, so there is a valid case for the brackets being there. The reasoning for this patch is, that in ASP.NET there is an integrated type (IFormFileCollection) that can receive multi-part/form-data-uploads. When using it, the file list is automatically mapped to the variable with a name that matches name property. This mechanism fails, because the variable name contains brackets.

There is a workaround for ASP.NET, but that requires maintaining a "manual" extraction of the files from the incoming request or creating a middle ware. Using a framework provided method is less work, more reliable and secure. Especially because we're dealing with binary file uploads here.

[artf]

Thanks @merlinschumacher appreciate the detailed reasoning of the PR