fix: update MIME type for tar.gz file acceptance in dropzone (#1862)

This solves an issue that allowed the users to import files .gz.
We make the drop zone allow only .tar.gz files.

Resolves https://github.com/openedx/frontend-app-authoring/issues/1386

src/import-page/file-section/FileSection.jsx

@@ -41,7 +41,7 @@ const FileSection = ({ intl, courseId }) => {
                   handleError,
                 ))
               }
-              accept={{ 'application/gzip': ['.tar.gz'] }}
+              accept={{ 'application/x-tar.gz': ['.tar.gz'] }}
               data-testid="dropzone"
               style={{ height: '200px' }}
             />