You should at the very least mention the existence of app.upload_set_config. Furthermore let me quote from the documentation:
UPLOADED_FILES_ALLOW This lets you allow file extensions not allowed by the upload set in the code.
so this overwrites or creates fallback default or what?
Question you should also be answering: How do i write the configuration of an UploadSet to the app? how do I set the allowed extensions for an UploadSet? How can i tell which UploadSets are configured if all i have is current_app. (i.e. from a blueprint in a separate file)
Also let me blow off some steam: This is the worst documented library I have encountered in my entire life. Please, if you want people to use it, fix this first before any further features or bug fixes. At the moment it would be better to just remove it and so force people the code, which is a thousand times more informative than the text.