I followed the tutorial Creating a custom block type and I found a little problem, not sure if it is a bug or expected behaviour.
In the tutorial there is a part describing how to get the file MIME type through API. Watcher is set to watch changes of source.link. If I try to change the file - not upload, but select from previously uploaded files - the link value becomes false. I have to refresh the page in order to get the updated value of link. Other properties seems to respect the change of the source file - filename, url etc.
So when I change the file, the source.link is updated, but instead of new file link its value becomes false. If I upload a completely new file, everything works as expected.
EDIT: Ok, I realized this problem is related to the already known issue