When I open a MHTML file using telegram, android browser downloads it whereas when it is opened up using some file manager, browser shows up webpage successfully.
The way telegram is storing files in internal storage and firing content url to open seems improper.
For e.g. Solid explorer's content url is content://pl.solidexplorer2.files/storage/emulated/0/Download/Confusing concepts of Enthalpy %E2%80%93 Telegraph (1).mhtml
whereas for telegram it is content://org.telegram.messenger.web.provider/media/Telegram/Telegram%20Documents/5_6150024775642121230.mhtml
This could be the case even with other file formats also. An alternative way would be to provide a open with dialog apart from system dialog.