Able to reproduce on reported version 60.0.3112.113, latest canary 63.0.3219.0 and on latest stable 61.0.3163.91 with the below mentioned steps on Ubuntu 14.04, windows 7 and Mac 10.12.6.
1. Loaded the extracted attached file into chrome://extensions.
2. Opened an Incognito window and tried to download.
3. On the download bar, got Failed- No file message.

This seems to be a Non-Regression issue seen from M-50[50.0.2624.0]. Hence, marking it as Untriaged.

Fun!  This is interesting.

I suspect the reason this happens is because the download request is originating from the extension process, which is an on-the-record process for spanning-mode extensions, which are the default.  Then, when we try to download a file from it, we fail, because there's a mismatch in expectations there (I haven't tracked down the exact spot yet -
 still wading through download code).

I've confirmed that if you set the incognito mode of the extension to "split", so that the process that initiates the download is an off-the-record process, the extension works in both incognito and on-the-record windows.

Assigning to benjhayden@ as owner of the downloads API, and cc some process gurus in case they have ideas or are interested.

I've moved off of downloads and forgotten this code. Redirecting to dtrainor.

xingliu@ can you take a look?

+qinmin@ for his recent work on the extensions API.

This can be repro on linux.

The URL for extension in incognito mode currently uses "blob:chrome-extension//" schema, where network stack yields FILE_NOT_FOUND error for some reason.

Log:
:download_request_core.cc(392)] OnResponseCompleted() { this=0xcfe633fe790  url_ = "blob:chrome-extension://dnpfdogidiapommpboahbmkljooanhgh/5e935d5f-25d9-407f-a9c0-f254c4053f78" } status.status() = 3 status.error() = -6 response_code = 0

Comment #3 is probably correct, the extension blob in incognito is saved in normal profile's blob storage, but the download system and url request are using incognito profile, and fails to find the blob in incognito profile's blob storage in:
https://chromium.googlesource.com/chromium/src/+/68.0.3415.1/content/browser/loader/resource_dispatcher_host_impl.cc#2117

Notice that the blob storage context is glued to a resource context and browser context as UserData.

It's very hard to fix in download code, that even I can find the blob data handle from normal profile in incognito mode, the download result will appear in the normal window instead of incognito window. So I guess the correct solution is probably to make the extension process uses incognito profile?