Add ability to attach thumbnails to image uploads

[masterofllamas]

Description: When image files are uploaded using this client, thumbnails do not show up for them on the web. The SDK docs say the upload client has to attach generated thumbnails when uploading an image. Is it possible to add this functionality? Apparently there are example applications in the SDK code to show how to do that.

Actual Behavior:

Thumbnails are not generated for image uploads

Expected Behavior:

There should be a way to attach generated thumbnails to image uploads

Steps to Reproduce:

1. Upload an image to a folder using this client
2. Browse to the folder in the mega web interface
3. Notice the image file doesn't show a thumbnail. If you open it inside the browser, then it will generate a thumbnail, but this is impractical when uploading hundreds of images.

MegaApiClient Version:
1.10.3

[gpailler]

I implemented recently DownloadFileAttribute() but its counterpart doesn't exist.
I will check if it can be done in the next version of the library