Incorrect mime types, e.g. image/jpg, should be image/jpeg

[tuomassalo]

Describe the bug

If given e.g. https://example.com/foo.jpg, formatEnclosure gives type="image/jpg" instead of the correct MIME type image/jpeg.

I haven't checked which clients actually care about this.

To Reproduce

This behavior is already visible in the tests:

feed/src/__tests__/__snapshots__/rss2.spec.ts.snap

Line 34 in fd77835

<enclosure url=\\"https://example.com/hello-world.jpg\\" length=\\"0\\" type=\\"image/jpg\\"/>

Expected behavior

The generated feed should contain:

<enclosure ... type="image/jpeg"/>

Actual behavior

The generated feed contains:

<enclosure ... type="image/jpg"/>

Versions (please complete the following information):

• feed: 4.2.2

Additional context

The code at https://github.com/jpmonette/feed/blob/master/src/rss2.ts#L215-L223 returns the MIME type as image/[file_extension], which is correct for e.g. .png and .gif, but incorrect for e.g. .jpg and .svg.