You should be presented with options for the same.
If you want to reset a specific file type then choose the option at the end labelled as "Choose default apps by file type". Screens attached for your reference:
1. Settings
2. Apps
3. Default Apps
The options in red color boxes are what you are looking for, I suppose :D
Greetings,
Navigate to settings>Apps>Default Apps
You should be presented with options for the same.
If you want to reset a specific file type then choose the option at the end labelled as "Choose default apps by file type". Screens attached for your reference:
1. Settings
2. Apps
3. Default Apps
The options in red color boxes are what you are looking for, I suppose :D