Dear User, there might be 3 reasons it could happen.
1. Please check the online application you have selected to convert the file.
2. If you're in the right application please check the extension of the file you have selected.
3. After selecting the file with the correct extension please check the size of the file, it should not exceed more than 5MB.
If still you're unable to add the file after following the above steps, please contact our support team for further help.