Error - The file type is not allowed - file cannot be uploaded

Question

I am creating an online digital photo store using PhocaCart 3.3.0 on a Joomla 3.9.10 site with a Gantry5 template that will provide both JPEG and DNG (digital negatives) files for sale.

The images will be store in a folder outside of public_html to prevent directory browsing and pirating photos.

I have added "image/x-adobe-dng" to legal MIME types, and added dng and DNG to both Legal Extensions (File Types) and Legal Image Extensions (File Types) and still I cannot upload any .dng files... and yes, I did increase my max upload file size to accommodate the larger .dng files - what am I missing?

Even with everything I tried, I am still getting "Error: The file type is not allowed - file cannot be uploaded".

Is it a PHP.ini setting at the server level, is it a Joomla setting, or the component that is restricting the .dng file upload?

Joomla 3.9.10 PhocaCart 3.3.0 PHP 7.3