Here's a simple replacement to Flask's send_file that supports 206 partial content via byte ranges (and a function that adds the appropriate headers to all responses). It doesn't fully wrap send_file, if you care about all the other arguments and mirroring the exact behaviour, it needs a bit more work, but should suffice for 99% of uses.

[update 31/8: sorry there was a bug in this which caused strange things with certain Chrome versions, it's now been fixed]

import mimetypes
import os
import re

from flask import request, send_file, Response

@app.after_request
def after_request(response):
    response.headers.add('Accept-Ranges', 'bytes')
    return response


def send_file_partial(path):
    """ 
        Simple wrapper around send_file which handles HTTP 206 Partial Content
        (byte ranges)
        TODO: handle all send_file args, mirror send_file's error handling
        (if it has any)
    """
    range_header = request.headers.get('Range', None)
    if not range_header: return send_file(path)
    
    size = os.path.getsize(path)    
    byte1, byte2 = 0, None
    
    m = re.search('(\d+)-(\d*)', range_header)
    g = m.groups()
    
    if g[0]: byte1 = int(g[0])
    if g[1]: byte2 = int(g[1])

    length = size - byte1
    if byte2 is not None:
        length = byte2 - byte1
    
    data = None
    with open(path, 'rb') as f:
        f.seek(byte1)
        data = f.read(length)

    rv = Response(data, 
        206,
        mimetype=mimetypes.guess_type(path)[0], 
        direct_passthrough=True)
    rv.headers.add('Content-Range', 'bytes {0}-{1}/{2}'.format(byte1, byte1 + length - 1, size))

    return rv