basically,there is a video how to integrate telegram:
All form elements (submitted data) should be send to the telegram.
I tested it now with file upload, and images is attached to the telegram.
Please make sure that
1) The image element isn't in the exlude list in the telegram setup
2) more important: that the "Use Url" checkbox isn't checked in the file upload element => advanced tab because this would transform the path to the file from a local server, to a webpath (using https://...) and then the file cannot be transferred under certain server setups.