This particular issue really stuck with me today. While working on a different CSV fix, I realized what is most likely giving you trouble here. Some web servers or CSV file creators can mis-apply the MIME type on a CSV file. WordPress itself won’t accept a CSV file with a MIME type other than text/csv, where it could easily be text/plaintext.
To address this, I’ve added a modification to our plugin that hooks in and expands the flexibility here. That should be available in an upcoming maintenance release.
If you are in need of this sooner, let me know and I can package up the fix as a temporary snippet/plugin for you.