Steps to reproduce the issue

Add mp4 to legal extensions and MIME types and try uploading a video using Media Manager. The video I am trying to upload is smaller than my PHP upload size limit and the joomla max size limit in global config. Note that this issue doesn't occur if I set restrict uploads to "NO".

Expected result

Successful upload.

Actual result

Error: Not a valid image

System information (as much as possible)

Joomla v3.9.2
PHP upload size is 25Mb
PHP v5.6.36
Zend engine v2.6.0
CentOS Linux 7 (core)

This is the video I am trying to upload:
big_buck_bunny.mp4.tar.gz

Additional comments

I am Super User
Joomla's System Information -> File Permission are all set to writable
Both the Image path and the File path are set to images.
Other media types such as images and documents upload with out errors.