It is important that the data you would like to upload to Explorer is on the first sheet in your Excel file. Make sure that the column you want to analyse
1. Contains a Header such as "text"
2. Does not contain formulas, as we can't guarantee that Explorer interprets them in the right way. If the column contains formulas, you should save your file as a CSV file (see below) to ensure that the data to be analysed actually contains text.
You can use a tool like http://csvlint.io/ to check if your file is a proper CSV. Another option is to upload the file to Google drive and open it as a google spreadsheet and then download it as a CSV (File → Download as → CSV). A third option is to open your original file in LibreOffice and save as CSV (this has the added benefit of enabling you to change the encoding if you want). All of these options give you good CSV files that adhere to the standard about which you can read more here: https://en.wikipedia.org/wiki/Comma-separated_values.
If you export from Excel you should note that you need to make the file comma-separated and not semicolon-separated.