Thanks for your questions and for the effort your put into working through the IPTC/EXIF and Custom Field mapping, which can be confusing at first.
Your second question asks “is there an option to strip the caption to the filename’s last few letters?” The answer is “yes” and you can find this and other options in the “Field-level option/format values” section on the Settings/Media Library Assistant Documentation tab. The “substr” option will handle your immediate needs.
I assume you have added something like template:[+file_name+]
to the EXIF/Template textbox in your Caption mapping rule. You can change that to template:[+file_name,substr(-4)+]
to get the last four characters of the file name, e.g., “.jpg”.
I am marking this topic resolved, but please update it if you have problems or further questions regarding option/format values, Content Templates or Caption field mapping rules. Thanks for your interest in the plugin.