Not a CSV file, creating UTF-8 files for import.
The CiviCRM import system requires csv files encoded in UTF-8 to correctly import 'accented' characters.
If you get an error message saying that your import is 'not a csv file' even though it is, then it's likely that you have accented characters and don't have UTF-8 encoding.
Although CiviCRM says that you can easily use csv files exported from Excel; Excel can't create UTF-8 encoded csv files. You need to go through an extra step to change the coding.
The simplest route that I have found is to save the csv file from Excel, then open it in Microsoft Word and Save as a 'Text' file (remembering to keep the .csv suffix). When you click Save an encoding box opens and you can choose 'Other encoding | Unicode UTF-8'.