Starting with 5.0.1 version WordPress introduced an additional security check, where file’s MIME type must match extension. So an attempt to upload any executable file file with .jpg extension will fail with:
Sorry, this file type is not permitted for security reasons
This can be a real pain when trying to upload certain ebook format, because this is exactly the case — a MIME-type conflicted with extension (at least as seen by WordPress). For example: Kindle’s .azw3 files declares itself (content; look inside) as
BOOKMOBI. Or .epub format which has
application/epub+zip as MIME-type and being a modified version of .zip file or actually a regular .zip file with some predefined content.
There are some ways to workaround this (discussed here), however, not all of them works, sorry!Read More “Uploading MIME-conflicted ebooks and files to WordPress”