When you drag and drop photos, Mailplane optimizes them to make them smaller and you can control the optimization in the Photos preference page.
But, sometimes you need to send a picture without any optimization: you want to leave size, file type, and picture quality untouched. The latest release now allows you to just press the command key while dropping the file. The mouse cursor will change to "Link" instead of "Copy" to indicate that no optimization will be performed.
Here is how it looks like:
I dragged a photo from Finder to Mailplane.
No key pressed: Optimize the picture according to your preferences
Command-Key pressed: disable all optimizations, leave the picture untouched.