Fun with Python and mimetypes! Works in Python 2.7.x.

python.mimetypes.py

import mimetypes
import os


def find_files(root, accepted_types=None):
    """Walk a path and find files that are a specific type, regardless of extension.
    
    :param root: Path to start walking for files.
    :type root: str
    :param accepted_types: (optional) List of acceptable mimetypes to match.
                           (default: None)
    :type accepted_types: list
    :return: List of full file paths that match the mimetypes.
    :rtype: list
    """
    if accepted_types is None:
        accepted_types = [
            'video/mp4',
            'video/x-msvideo'
        ]

    results = []
    for root, root_subdirs, root_files in os.walk(root):
        # Skip empty directories.
        if not root_files:
            continue

        for root_file in root_files:
            file_path = os.path.join(root, root_file)

            if os.path.isdir(file_path):
                continue

            mime_results = mimetypes.guess_type(file_path)

            if mime_results[0] not in accepted_types:
                continue

            results.append(file_path)
    return results