When creating a new Media Source the dialog instantly gives an error on the "Source Type" field.
Create a new Media Source
It starts with an error because there is no default "Source Type" selected.
It should not give an error immediately. Give the error when the Media Source is being saved or add an option to set a default source type. Preferable defaulting to "sources.modFileMediaSource".
MODX 2.6.*, 3
To avoid conflicts during merging fix should be applied to 3.x version where media sources already refactored.
Confirmed in MODX 3

To fix this issue it's best to introduce a new setting for the media source source type: default_media_source_type with a default value of sources.modFileMediaSource.
This way users can change the default source type.
Can we rename "sources.modFileMediaSource"? Now it confused

@meshkov thanks for taking time sharing your suggestion, can you open a new issue for your suggestion because this issue is closed and it might go unnoticed.
@JoshuaLuckers done. https://github.com/modxcms/revolution/issues/14818