feat(attachment): recognize .log files as text/plain mimetype (#44509)

Should fix #38189

As I understand the guess of mimetype occurs here:
https://github.com/getsentry/sentry/blob/184fa3bfd28266e7bd91abe4b2b3bdf6d11825a2/src/sentry/models/eventattachment.py#L53-L61

And by default `mimetypes.guess_type("test.log")` would return `(None,
None)` instead of `('text/plain', None)`, ending up with
`application/octet-stream` as the MIME type for `.log` files in the API.

And then right here the `.log` file would have been previously guessed
as non-previewable in the UI:

https://github.com/getsentry/sentry/blob/184fa3bfd28266e7bd91abe4b2b3bdf6d11825a2/static/app/components/events/eventAttachments.tsx#L38-L56

So this change should let `mimetypes` detect this file type properly and
fix the aforementioned issue (that is assuming the `Content-Type`
header, wherever it comes from, wouldn't override it either...).

### Legal Boilerplate

Look, I get it. The entity doing business as "Sentry" was incorporated
in the State of Delaware in 2015 as Functional Software, Inc. and is
gonna need some rights from me in order to utilize my contributions in
this here PR. So here's the deal: I retain all rights, title and
interest in and to my contributions, and by keeping this boilerplate
intact I confirm that Sentry can use, modify, copy, and redistribute my
contributions, under Sentry's choice of terms.

src/sentry/runner/settings.py

@@ -109,6 +109,7 @@ def configure(ctx, py, yaml, skip_service_validation=False):
         ("application/x-font-ttf", "ttc"),
         ("font/opentype", "otf"),
         ("image/svg+xml", "svg"),
+        ("text/plain", "log"),
     ):
         mimetypes.add_type(type, "." + ext)