After upgrading an Umbraco installation from 3.0 to 126.96.36.199, I got the error:
"The create dialog for "Media" does not match anything defined in the "~/umbraco/config/create/UI.xml" while trying to upload images.
To fix this, simply open UI.xml located in: /umbraco/config/create/
and rename it to uppercase:
Restart IIS and problem should be solved.