All images uploading... except JPG files
I had this same issue and resolved it (at the suggestion of Craft CMS support) by enabling ImageMagick in the php.ini and restarting MAMP PRO.

Per the MAMP PRO 3.0 user guide:

The ImageMagick PHP module is included by default, but you must enable it inside the php.ini template file. Remove the ‘;’ from the following line.
;extension=imagick.so