Fix upload_file not seeking streams back
Fix upload_file not seeking streams back

This would cause issues in _cache_media since utils.is_image fails
in the second pass (it respects the stream's position, and the user
may rightfully pass a stream that should be read only from one pos).

LonamiWebs/Telethon,LonamiWebs/Telethon,LonamiWebs/Telethon,expectocode/Telethon,LonamiWebs/Telethon