Which file manager exactly?  There is a "File Manager plus" and a "File Manager Plus" in the app store.

By Flasklight + Clock

Sorry for typo. Should be "Flashlight + Clock"

I tried and can reproduce it, however using the intent analyzer I'm seeing the intent generated by File Manager + to open an MHTML is:
Action: android.intent.action.VIEW
Categories: NONE
Type: application/octet-stream
Data: {the file url to the offlined mhtml file}

Extras: NONE

So can this be related to the octet-stream thing that was discussed before?

Also I can see for some MHTMLs (like the one I saved from amazon.com), File Manager + cannot get a MIME type for it, and the system will ask to open it as [Text | Image | Audio | Video | Other].

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/4e5405f4909611b9fe80404dbea0c2825846d14d

commit 4e5405f4909611b9fe80404dbea0c2825846d14d
Author: Jian Li <jianli@chromium.org>
Date: Tue Apr 24 18:46:00 2018

Detect application/octet-stream type for intent with MHTML file URL

"application/octet-stream" type may be passed by some apps that do
not know about MHTML file types. So when we check if an intent is
to open MHTML file, we allow this type and let the following file
extension check to find out if the file URL is for MHTML file.

Bug:  831767 
Change-Id: Ide23ebbfd4a4fd9d47e8a06003ea2cd66338a354
Reviewed-on: https://chromium-review.googlesource.com/1024506
Commit-Queue: Jian Li <jianli@chromium.org>
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553215}
[modify] https://crrev.com/4e5405f4909611b9fe80404dbea0c2825846d14d/chrome/android/java/src/org/chromium/chrome/browser/IntentHandler.java
[modify] https://crrev.com/4e5405f4909611b9fe80404dbea0c2825846d14d/chrome/android/javatests/src/org/chromium/chrome/browser/IntentHandlerTest.java

Verified with 68.0.3410.0.

This bug requires manual review: M67 has already been promoted to the beta branch, so this requires manual review
Please contact the milestone owner if you have questions.
Owners: cmasso@(Android), cmasso@(iOS), kbleicher@(ChromeOS), govind@(Desktop)

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e2e233129ba8f46eef20ca735042858bfe04dc0d

commit e2e233129ba8f46eef20ca735042858bfe04dc0d
Author: Jian Li <jianli@chromium.org>
Date: Tue May 01 20:26:33 2018

M67: Detect application/octet-stream type for intent with MHTML file URL

"application/octet-stream" type may be passed by some apps that do
not know about MHTML file types. So when we check if an intent is
to open MHTML file, we allow this type and let the following file
extension check to find out if the file URL is for MHTML file.

TBR=jianli@chromium.org

(cherry picked from commit 4e5405f4909611b9fe80404dbea0c2825846d14d)

Bug:  831767 
Change-Id: Ide23ebbfd4a4fd9d47e8a06003ea2cd66338a354
Reviewed-on: https://chromium-review.googlesource.com/1024506
Commit-Queue: Jian Li <jianli@chromium.org>
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#553215}
Reviewed-on: https://chromium-review.googlesource.com/1038227
Reviewed-by: Jian Li <jianli@chromium.org>
Cr-Commit-Position: refs/branch-heads/3396@{#426}
Cr-Branched-From: 9ef2aa869bc7bc0c089e255d698cca6e47d6b038-refs/heads/master@{#550428}
[modify] https://crrev.com/e2e233129ba8f46eef20ca735042858bfe04dc0d/chrome/android/java/src/org/chromium/chrome/browser/IntentHandler.java
[modify] https://crrev.com/e2e233129ba8f46eef20ca735042858bfe04dc0d/chrome/android/javatests/src/org/chromium/chrome/browser/IntentHandlerTest.java