Hello, 

I test this way of doing.

I found that (v17) we have to remove de json extension of the file we want to import.

The mime type will be text/plain after the opération but it runs.

I found also that you can not import a file without replacing a file that exist, neither duplicate an attachement. You have to create a virgin file, and replace the file by the one you to import.

Yes but its not very user friendly to get to.

spreadsheets are basically an attachment in Odoo. (model ir_attachment)

If you enable developer mode and go to settings > Technical > Data Structure > Attachments

You will see all your attachments. If you recently create a spreadsheet it will be at the top of the list.

Look for the name of the spreadsheet and open the attachment.

It will have a File Content field (datas field name in database) with the json file for the spreadsheet. 

I downloaded the file , modified it , cleared the one on the File Content field and reuploaded it.