Unable to reproduce the issue on reported chrome version 67.0.3396.99 using Windows 10 . Attaching screen-cast for reference.
Steps: 
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1. Launched reported chrome 
2. Navigated to given URL "http://mime.ritey.com/"
3. uploaded ' text/csv ' files 
As we have seen that The MIME type for your file is: "text/plain" and "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet" for both files.

@Reporter: Could you please review the attached screen-cast and confirm if anything being missed here and Request you to retry this issue with fresh profile without any extensions/apps or reset all the flags and let us know if issue still persists.

Thanks.!

Deleted: 862565.mp4 3.9 MB

	862565.mp4 3.9 MB View Download

I reviewed your screencast and everything seems the same I did except for the conclusion. I used a freshly installed Chrome without a profile and without any add-on or extension. I verified the problem on other computers used by my colleagues.

Here is my screencast.

Deleted: ChromeBUG.mov 10.4 MB

	ChromeBUG.mov 10.4 MB View Download

Thank you for providing more feedback. Adding the requester to the cc list.

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

As per comment #3 tested and Unable to reproduce this issue on reported chrome version 67.0.3396.99 using Windows 10. As we getting same as per comment #2. Hence adding Blink>Forms>File and requesting team to have a look on it for further triage it.

Thanks..!

Extension->MIME type mapping is done in net/.

MIME type mappings actually come from the local system configuration in the registry (With a fallback list if we can't find anything, I believe).  Not sure if this is worth re-visiting.

As the issue is not reproducible from our end (...C#2 & C#5) adding label "TE-NeedsTriageHelp" and requesting someone from respective team to have a look into this and help in further triaging.

Thanks!