don't report error on unknown mime type from S3

[syphar]

I saw this sentry error,

from (for example) serving http://docs.rs/crate/mach_object/0.1.3/source/test/helloworld.universal, which raises the error "could not parse mime from storage object".

IMO we don't need this error, defaulting to application/octet-stream is totally fine.

[Nemo157]

The question I have is how did an invalid mimetype get into S3, and I probably found the answer with a little more looking, it's an old crate built in 2017 back when https://docs.rs/magic was used for mimetype detection, I'm not surprised it may have put some invalid data into the database back then.

[Nemo157]

(This does make me question our current default of text/plain when mime detection fails while adding a file though)

docs.rs/src/storage/mod.rs

Lines 549 to 551 in c927eac

	let mime = mime_guess::from_path(file_path.as_ref())
	.first_raw()
	.unwrap_or("text/plain");

[syphar]

I moved both comments / questions into an issue: #2050