fix: add e-book MIME types

[PixelQuasar]

Q	A
Bug fix?	yes
New feature?	no
BC breaks?	no
Deprecations?	no
Tests pass?	not run — data-only change, verified manually (below)
Fixed tickets	#6148
License	MIT

Adds four e-book formats to the MIME table in lib/API/Serve.js. They are
currently absent, so pm2 serve labels them text/plain: browsers render the
bytes instead of downloading the file, and readers that dispatch on
Content-Type do not recognise it.

'azw3': 'application/vnd.amazon.mobi8-ebook',
'epub': 'application/epub+zip',
'fb2':  'application/x-fictionbook+xml',
'mobi': 'application/x-mobipocket-ebook',

Entries are inserted in the existing alphabetical order. application/epub+zip
is registered with IANA; the other three are the de facto values.

Purely additive — it only affects extensions that previously fell through to
the default, so nothing that works today changes. In particular the text/plain
fallback itself is untouched.

Verification

Ran the patched Serve.js directly, same way PM2 invokes it:

book.epub      → application/epub+zip
book.fb2       → application/x-fictionbook+xml
book.mobi      → application/x-mobipocket-ebook
book.azw3      → application/vnd.amazon.mobi8-ebook
book.pdf       → application/pdf      (unchanged, already in the table)
note.unknown   → text/plain           (unchanged, fallback intact)

The last two lines are the point: existing behaviour is unaffected.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}