Solved : Unable to guess the MIME type as no guessers are available (have you enabled the php_fileinfo extension?).

Issues: 

1. Image doesn't load correctly or shows broken

2. File (i.e: csv, xlsx etc.) will not be downloaded or cannot be exported

Solution:

in WHM:

1. Navigate to Software -> EasyApache4
2. Click the button that says Customize next to Currently Installed Packages
3. Click PHP Extensions
4. Search for Fileinfo in the search bar and select the extensions you would like to install
5. Click Review
6. After looking over the proposed changes, scroll to the bottom of the screen and click provision.

In cPanel using PHP Selector (if not works even installed in WHM):

In cPanel using ini file (if not works even installed in WHM):

Other:

1. Install PHP FIleInfo Extension (if not installed)
2. Enable php_fileinfo in php.ini 

extension=php_fileinfo.dll (Windows)

extension=php_fileinfo.so (Linux)