I think the easy way is to use the (S)FTP module for that. Make two types. One for images and one for the rest. Let the images become Mendix image documents and let the rest become the standard filedocuments. Once you have that you can use microflows again to connect the images and documents again. If you can agree on a kind of filename standard you can use that to connect them again to other imported data.
I have found another easy way that is copy all the images and file documents on my desktop and create two entities for image and file document and then use dropzone widget from appstore where I can multi select or just drag and drop all the images / file documents to upload them into Mendix system. The good thing is both images and file documents have same unique names and hence I can easily connect them back again.