JPEGmini uses the standard baseline JPEG format, which is by far the established market leader in the image compression space.
Newer formats such as JPEG2000, JPEG-XR and Web P have not gained significant market share yet.
Choose Transmit → Preferences → Advanced, then click the Advanced Server Settings button.
If you're using an earlier version of Mac OS X, you'll need to stick with Transmit 4.4.3 for now.
You are welcome to test the quality on your own photos: After optimizing them with JPEGmini, compare them side by side at 100% zoom (“Actual Size”), where each pixel on the screen corresponds to a pixel in the photo. Since JPEGmini does not alter the perceptual quality of your photos, you can print them at the same size you would print your original photos, and the prints will look exactly the same.
Watch JPEGmini Pro Being tested under the loupe https://youtu.be/xdnd N5kobdg Yes, JPEGmini accepts standard JPEG files as input, and outputs standard JPEG files as well.
Fortunately, this is not an issue that has no solution whatsoever.