file.icon.theme.pcss.css

.field-icon-file_upload {
  background-image: url(../../../misc/icons/55565b/file_upload.svg);
}

File

core/modules/file/css/file.icon.theme.pcss.css

.field-icon-file_upload {
  background-image: url(../../../misc/icons/55565b/file_upload.svg);
}