Hi Horst,
.Net Agent can only instrument .Net Applications. Since in your case the files are just hosted on IIS, no .Net runtime is involved when these files are requested and hence you do not see any BTs.
I don't think we can get any information by instrumenting the site hosting these mp4 files.
I think we will need to capture this info via EUM only until it can be captured at SharePoint Server side.
One idea can be to add Custom User Data to a Page Browser Snapshot when ever user requests the video
https://docs.appdynamics.com/display/PRO42/Add+Custom+User+Data+to+a+Page+Browser+Snapshot
Let me know if you have any questions or I mis understood your question.
Thanks,
Raunak