How does HTTP file upload work?

When I submit a simple form like this with a file attached:

<form enctype="multipart/form-data" action="http://localhost:3000/upload?upload_progress_id=12344" method="POST">
<input type="hidden" name="MAX_FILE_SIZE" value="100000" />
Choose a file to upload: <input name="uploadedfile" type="file" /><br />
<input type="submit" value="Upload File" />
</form>

How does it send the file internally? Is the file sent as part of the HTTP body as data? In the headers of this request, I don’t see anything related to the name of the file.

I just would like the know the internal workings of the HTTP when sending a file.