Currently, when selecting a part that is not displayable, aerc offers to save or pipe it. Chosing "Save" opens a :save prompt. This prompt should include the filename from the part list, so I can just hit enter unless I want to change the name.
Aerc already uses the filename indicated by the message part. While it's not displayed in the prompt, it allows you to quickly do something like
:save ~/downloads/if you are manually typing or if you do that frequently you can set the default path and then doing:savewithout any arguments does exactly what you want.If aerc would prefill the filename into the prompt, you would first have to navigate backwards with say the arrow keys before you could choose a directory, which is annoying.
I don't think that this would be an improvement.