I saved Wikipedia's landing page as an offline page and inspected the intents generated by ADM (from Android O) and ASTRO File Manager, an app that successfully opens the file with Chrome Canary. These are the differences I found:

Type: 
* ADM: multipart/related
* ASTRO: message/rfc822

Data:
* ADM: content://com.android.providers.downloads.documents/document/3453
* ASTRO: content://com.metago.astro.filecontent/file/storage/emulated/0/Download/Wikipedia, the free encyclopedia.mhtml

Flags:
* ADM: FLAG_ACTIVITY_FORWARD_RESULT, FLAG_ACTIVITY_PREVIOUS_IS_TOP, FLAG_GRANT_READ_URL_PERMISSION
* ASTRO: FLAG_ACTIVITY_FORWARD_RESULT, FLAG_ACTIVITY_PREVIOUS_IS_TOP

This is caused by ContentResolver.getType() returning "application/octet-stream", instead of "multipart/related" in Oreo.

It is called at https://cs.chromium.org/chromium/src/base/android/java/src/org/chromium/base/ContentUriUtils.java?type=cs&l=120.

Android bug was filed: http://b/73736356

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/74b656c180ca06d86caef3cb42df977cdafadf76

commit 74b656c180ca06d86caef3cb42df977cdafadf76
Author: Jian Li <jianli@chromium.org>
Date: Wed Mar 21 00:25:34 2018

Pass intent type in extra headers for content URI

On Oreo, ContentResolver.getType() returns "application/octet-stream"
for content uri that is sent from Android Downloads Home, instead of
the registered MIME type. To work around this, we pass the intent
type in extra headers such that content request job can get it.

Bug:  813906 
Change-Id: I86c87b0458dc5b4311bcd7ae7248af1ea9df7eaf
Reviewed-on: https://chromium-review.googlesource.com/961560
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Reviewed-by: Maria Khomenko <mariakhomenko@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Commit-Queue: Jian Li <jianli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544602}
[modify] https://crrev.com/74b656c180ca06d86caef3cb42df977cdafadf76/chrome/android/java/src/org/chromium/chrome/browser/IntentHandler.java
[modify] https://crrev.com/74b656c180ca06d86caef3cb42df977cdafadf76/chrome/android/javatests/src/org/chromium/chrome/browser/IntentHandlerTest.java
[modify] https://crrev.com/74b656c180ca06d86caef3cb42df977cdafadf76/content/browser/android/url_request_content_job.cc
[modify] https://crrev.com/74b656c180ca06d86caef3cb42df977cdafadf76/content/browser/android/url_request_content_job.h
[modify] https://crrev.com/74b656c180ca06d86caef3cb42df977cdafadf76/content/browser/android/url_request_content_job_unittest.cc

Verified on Chrome:67.0.3378.0 Device:Nexus 6/MRA59G with the steps mentioned in C#0