As per your code, you have var file = document.getElementById('tool-importSVG');
But inside the event listening function, again you are using var file = files[i];
It means you are redefining the file variable. I would suggest you to rename that variable.
Now coming to your question:
You can use a bootstrap button to upload files:
<span class="btn btn-success fileinput-button">
<i class="glyphicon glyphicon-plus"></i>
<span>Add files...</span>
<input type="file" name="files[]" multiple="">
</span>
It will look like below:
CSS:
.fileinput-button input {
position: absolute;
opacity: 0;
-ms-filter: 'alpha(opacity=0)';
direction: ltr;
cursor: pointer;
margin-top: -25px 0 0 0;
}
You can change the color as you want :)