Fetch API upload function with RegEx validation

const uploadFunction = event => {
    const fileInput = event.target;
    const files = event.target.files
    const path = event.target.value;
    
    const allowedExtensions = /(\.jpg|\.jpeg|\.png|\.gif)$/i;
              
    if (!allowedExtensions.exec(path)) {
        alert('Invalid file type');
        fileInput.value = '';
        return false;
    }
  
  
    const data = new FormData()
    data.append('file', files[0])
    
    fetch('/upload-file', {
    method: 'POST',
    body: data
    })
    .then(response => response.json())
    .then(data => {
    console.log(data)
    })
    .catch(error => {
    console.error(error)
    })
}