fix early bail when testing if string field should link to File node

[pieh]

mime@^1.0.0 used to return 'application/octet-stream' for unkown filenames - this was changed in mime@^2.0.0 and now it returns null ( ref: broofa/mime#139 )

When we upgraded mime and didn't adjust this caused every string field to go through quite expensive File nodes lookup - causing very long schema generation in some cases.

Closes #6467

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[KyleAMathews]

Looks like there's a few other places we make this check? https://github.com/gatsbyjs/gatsby/search?q=%22application%2Foctet-stream%22&unscoped_q=%22application%2Foctet-stream%22

[pieh]

Yeah, I checked that before and just didn't update it because it would work:

gatsby/packages/gatsby-source-filesystem/src/create-file-node.js

Lines 43 to 49 in 27a4f4b

	const mediaType = mime.getType(slashedFile.ext)
	internal = {
	contentDigest,
	type: `File`,
	mediaType: mediaType ? mediaType : `application/octet-stream`,
	description: `File "${path.relative(process.cwd(), slashed)}"`,
	}

this will actually keep v1 behaviour of setting application/octet-stream for File nodes if mime.getType will report null, but probably noone will test against either null or application/octet-stream

But I guess if it doesn't matter let's not diverge from current mime, so I'll just update it too

[KyleAMathews]

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[m-allanson]

👍 thanks @pieh