Adjust temporary files

[catap]

Rigth now it creates something like:

.../Perota Chingo/Un viajecito/.12 Rie Chinito.flac64u8gm5u.beets

where no separatio between temp name and file extention, and it makes quite hard to read it.

So, this changes adjust name to

.../Perota Chingo/Un viajecito/.12 Rie Chinito.flac.64u8gm5u.beets

This was one of forgotten fixup from https://github.com/beetbox/beets/commit/8d50301be58463ac860cafaf898670fccb525bbe

Description

Fixes #X.

(...)

To Do

• [ ] Documentation. (If you've added a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• [ ] Changelog. (Add an entry to docs/changelog.rst to the bottom of one of the lists near the top of the document.)
• [ ] Tests. (Very much encouraged but not strictly required.)

[Serene-Arc]

Hi, a changelog entry would be good for this, just on the off-chance that some plugin writer has coded some tests that are impacted by this change. If they suddenly get errors moving forwards, it would be good to have a quick reason why!

Otherwise, looks fine though, thanks for the contribution.