Some file types are reporting its MIME type different than it is associated with given file extension in WordPress database. Such file upload will be blocked for security reasons (.epub file in my case):
X.epub: Sorry, this file type is not permitted for security reasons
ALLOW_UNFILTERED_UPLOADS flag is used in WordPress in such situations. You should try it first. If you still have not satisfying results (i.e. you still can’t upload a file of certain type) then it may mean some MIME table changes made by either your plugin or theme or even a nasty bug in WordPress itself.
In all cases, using must-use plugin with some small filter should solve the problem.Read More “File upload blocked despite ALLOW_UNFILTERED_UPLOADS set to true”