Argument #1 ($mimeType) must be of type string, null given, called in /file.module in file\IconMimeTypes::getIconClass()

This same error message is discussed in a core issue ( 🐛 Drupal\file\IconMimeTypes doesn't handle NULL mimetypes Active ), but that seems to more about actually missing mime types. This one here appears to be caused by the way EE is handling it.

OK, I think it might be just a very stupid thing: I prepared a file data structure that I did not pass to the file create() method...
Could you try this fix and let me know if it works?

By the way, I never tried the file field plugin with data alteration operations! I'm not sure it will work in that case but who knows... :-p
From my perspective, you can not alter an external file you got from a URI but you can alter the URI you got. So, the code that might be missing is when saving data, the file field plugin should get the new file URI and report it to the text field used to map the URI. But I'm not sure it will be so easy. What if the URI field as also altered manually? Who wins? The value entered by the user or the one provided by the new file? Maybe, the behavior should be set on the file field mapper plugin settings? --> a new feature request? :-)

And for information (to people wondering how that works and did not find where it is explained), most of it is explained in this README.md. It could be improved though. Let me know what could be clarified.

It worked! Thank you.

Merged.

I created Feature Request ✨ Allow mapping of file URI to a field Active for adding to the URI field on creation and modification of files.

Specially, that's ✨ Allow mapping of file URI to a field Active (with the funky markup rendering).

Automatically closed - issue fixed for 2 weeks with no activity.