I don't see a way to avoid "scripting around it".
It's got to be text or image file types as perceived by FileMaker. If the files don't have a text file extension, FileMaker can't see them as text files for this import.
You'd need to use other means to import the files if you can't rename them. There are plug ins that list folder contents and you can use a get function to list the folder contents if the folder is located inside the documents folder. Once you have a file path to the folder and a list of files in the folder, a looping script can insert them one at a time.
I wish there was a way to extend the definition of text files in Filemaker.