Add config options for downloading

[jfvilas]

Currently, the only download option is just set an URL in 'filePreviewPath' for downloading files. Typically, when using SPA's, backend and frontend are served from different sites, so it is important to be able to "protect" backend endpoints. Normally, frontend apps send API keys using headers when invoking backend endpoints (header 'Authorization', for example).

This need is also applicable to the download endpoints.

It would be very interesting to be able to create a a "fileDownloadConfig" parameter where we can add such information (HTTP headers in general), the URL to use and other parameters.