I uploaded a rather large file that was saved using Notepad++. In the info line, the editor claimed the file was UTF-8, Windows (CR LF). Upon uploading, the site said “Failed to read… Are you sure it is in UTF-8 format?”
So I used the editor’s menu to convert the encoding to “UTF-8 with BOM”, and then tried again, with the same error.
Then I used the editor’s menu to convert the encoding to ASCII, and it uploaded successfully!
UPDATE:
The file did contain some strange characters. It was the same sequence of three characters in multiple places. The first was a Lambda, which did not appear to be converted to UTF-8 properly upon export from Excel, followed 0xC7 and 0xF4 characters. The last two are valid UTF-8 characters, which are variation C and o, respectively.
However, the file still could not be read after removing the lambda character, leaving the 0xC7F4. I still had to convert it to ASCII in order to upload it.