How to select file to attach to E-mail from "My Files"
It may be your file explorer isn't written well. You could try to use a different file explorer, such as ES File Explorer.

Or, try going about it from a different way. Instead of using your email program to look for an attachment, use your file explorer to 'share' the attachment, and choose to share it via your email app. It will add the file as an attachment and open your email app. As long as your email client is listed as a possible 'share' app, this should work.

If neither of those work, try getting a new email client. Since the email client isn't showing all of your file explorer apps when selecting to add an attachment, it may have written its own (poorly designed) app selector.