check file media type (mime) with javascript

get-media-type.js

function readBlob(blob) {
  return new Promise((res, rej) => {
    const reader = new FileReader()
    reader.onloadend = function() {
      res(reader.result)
    }
    reader.onerror = rej
    reader.readAsDataURL(blob)   
  })
}

async function getMediaType(blob) {
  if (blob.type) return blob.type
  blob = blob.slice(0, 4)
  const data = await readBlob(blob)
  const [,type] = data.match(/^data:(.+);/)
  return type
}

sample.html

<input type="file" id="fileInp" />

<script>
  fileInp.addEventListener("change", async (event) => {
    const [fileBlob] = event.target.files
    
    console.log(await getMediaType(fileBlob))
  })
</script>