Check out example codes for "file type input limit in html". It will help you in understanding the concepts better.

Code Example 1

<!-- Right approach: Use both file extensions and corresponding MIME-types. -->
<!-- (IE 10+, Edge (EdgeHTML), Edge (Chromium), Chrome, Firefox) -->
<input type="file"
 accept=".xls,.xlsx, application/vnd.openxmlformats-officedocument.spreadsheetml.sheet,application/vnd.ms-excel" />

Code Example 2

<!-- (IE 10+, Edge (EdgeHTML), Edge (Chromium), Chrome, Firefox 42+) -->
<input type="file" accept=".jpg,.png,.pdf" />

Learn ReactJs, React Native from akashmittal.com