Fetching external files (i.e. stored on Google Drive)

[Rustam]

Is your feature request related to a problem? Please describe.
As per #206 the external files are skipped.

Describe the solution you'd like
Check if external files can be downloaded. Consider file size of the external file as well, maybe have a threshold beyond which the external files will be skipped.

Additional context
Some example file fields (as per #206)

files: [
{
        "id" : "F12345678",
        "name": "foo bar",
        "title": "foo bar",
        "mimetype": "application/vnd.google-apps.document",
        "image_exif_rotation": 0,
        "filetype": "gdoc",
        "pretty_type": "G Suite Document",
        "user": "USER_ID",
        "mode": "external",
        "editable": false,
        "is_external": true,
        "external_type": "gdrive",
        "size": 0,
        "url": "",
        "url_download": "",
        "url_private": "attachments/F12345678-foobar",
        "url_private_download": "attachments/F12345678-foobar",
        // thumbnails are populated
}