I think this is a case of Numbers being too smart for it's own good (or, at least, for yours)...
When you select a group of files in the Finder and copy them, the Finder copies the data in a number of formats - as a list of file references (including paths), as a list of file names (plain text), and possibly even as the file contents.
When you paste that data, it's up to the destination application to determine what data to paste. Generally, applications choose the 'richest' format (the one with the most data) that they support. In this case, Numbers seems to paste any image data that it can identify, thinking that's 'more useful' than the list of names.
Think of it the other way around - some people would want the image data, and that's Numbers' assumption.
Unfortunately, that's not what you're looking for, and there's no way to tell the application to 'dumb down' the paste to the lowest common denominator (in this case, just file names).
The solution is to go through some intermediary app that doesn't understand the image data - which is why pasting into TextExit works because TextEdit can't process the image data, and the list of file names is all it can wean from the clipboard. When you re-copy that data, you just get the file names because that's all TextEdit knows.
As SGIII mentioned, there may be ways to automate this, but it's just replacing one step with one other step, so the benefits may not be worthwhile.
