How to replace post-upload text? Currently it shows filename and file size

This can be done with some custom CSS.

uploadcare--widget__file-name and uploadcare--widget__file-size are CSS classes that interest you.

Suppose you want the widget to show text label “Edit” instead of default values. Following style sheet snippet will do that for you:

.uploadcare--widget__file-name:before {
  content: "Edit"; 
  visibility: visible;
}

.uploadcare--widget__file-name {
  visibility: hidden;
}

.uploadcare--widget__file-size:before {
  content: ""; 
  visibility: visible;
}

.uploadcare--widget__file-size {
  visibility: hidden;
}