Have you tried to upload a file to your Wordpress website only to encounter an error reading, "Sorry, this filetype is not permitted for security reasons?" If yes, then you ran into the same problem that I recently had when trying to upload an .AI (Adobe Illustrator) file. Wordpress prevents some of the unpopular file types for security reasons-and while I totally buy it, it is a pain in my arse. After much research and inter web digging, I finally ran across a website that helped me out. I am sure that this is obvious to many people who know code, but I have no flipping' idea about anything dealing with that topic.  The video was short, I couldn't make it full screen, and she did not tell me what steps to follow in Wordpress. A little diligence and a hail mary helped me figure it out for myself. 1.  First of all, the code you will need to insert is below, so copy that to your clipboard.

function my_myme_types($mime_types){
    $mime_types['svg'] = 'image/svg+xml'; //Adding svg extension
    return $mime_types;
}
add_filter('upload_mimes', 'my_myme_types', 1, 1);

2. Next, go to your Wordpress Dashboard and scroll down to "Appearance" then to "Editor."

3. On the right hand side you will see a list of code pages to open-find the one that says, "Theme Functions (Functions.php)"

Once you click on it, you will reach a scary page that tells you not to edit the code unless you REALLY, REALLY, REALLY know what you are doing. Go ahead and click it. :) 4. Paste the snippet of code at the top but under the <?php. I added a space between the first and last lines. Note: This snippet of code was for a .SVG file type. Since I wanted to upload .AI files, I needed to copy and paste the line that says,

$mime_types['svg'] = 'image/svg+xml'; //Adding svg extension

and instead, I substituted ai anywhere it read svg. I would assume that you can keep doing this for any file type you need. You can see how I did it in the image above.

Once I followed these steps, I was able to upload my .AI file with no error. Let me know if it helped you too.