The Grey on the right side of the media page will disappear when you choose the chart you wish to upload

your selected chart will have a check mark on the top left of it

Don’t immediately hit “insert to post” at this stage

go to the right side of the page which now has the choices

Now on the dropdown menu to the right of “link to”….. highlight “media file” (don’t leave this as “none”)

then you can “insert to post”

you only need to do this one time and the setting will remain there