### Please describe the problem.
It seems that SHA256E, MD5E etc backends ignore file extensions longer than 4 characters, considering files with such extensions to have an empty extension.
But it's not uncommon to have longer extensions; e.g. .fasta and .fasta.gz files are common in bioinformatics.
Is it possible to remove this 4-character limit, or make it configurable?

### What steps will reproduce the problem?
(master_env_py27_v28) [12:37 PM /data/ilya-work/sw]$ cp c10.yyy c11.yyyy
(master_env_py27_v28) [12:37 PM /data/ilya-work/sw]$ git annex calckey c11.yyyy
SHA256E-s18841--9fd9a2607e019b7726c722d9d6f6171e6578f255bc60a0b79c525f8a3ffa05de.yyyy
(master_env_py27_v28) [12:37 PM /data/ilya-work/sw]$ cp c10.yyy c12.yyyyy
(master_env_py27_v28) [12:37 PM /data/ilya-work/sw]$ git annex calckey c12.yyyyy
SHA256E-s18841--9fd9a2607e019b7726c722d9d6f6171e6578f255bc60a0b79c525f8a3ffa05de

(master_env_py27_v28) [12:43 PM /data/ilya-work/sw]$ git annex calckey c10.yyyy.gz
SHA256E-s2168--21bb6c514473754cc49a455f45bc84961fe4fceb2cb0527ba2a1cfabdce6bf80.yyyy.gz
(master_env_py27_v28) [12:43 PM /data/ilya-work/sw]$ mv c10.yyyy.gz c10.yyyyy.gz
(master_env_py27_v28) [12:43 PM /data/ilya-work/sw]$ git annex calckey c10.yyyyy.gz
SHA256E-s2168--21bb6c514473754cc49a455f45bc84961fe4fceb2cb0527ba2a1cfabdce6bf80.gz

### What version of git-annex are you using? On what operating system?

git-annex version: 6.20180807-gebc1bb5
build flags: Assistant Webapp Pairing S3(multipartupload)(storageclasses) WebDAV Inotify ConcurrentOutput TorrentParser MagicMime Feed\
s Testsuite
dependency versions: aws-0.17.1 bloomfilter-2.0.1.0 cryptonite-0.23 DAV-1.3.1 feed-0.3.12.0 ghc-8.0.2 http-client-0.5.7.0 persistent-s\
qlite-2.6.2 torrent-10000.1.1 uuid-1.3.13 yesod-1.4.5
key/value backends: SHA256E SHA256 SHA512E SHA512 SHA224E SHA224 SHA384E SHA384 SHA3_256E SHA3_256 SHA3_512E SHA3_512 SHA3_224E SHA3_2\
24 SHA3_384E SHA3_384 SKEIN256E SKEIN256 SKEIN512E SKEIN512 BLAKE2B256E BLAKE2B256 BLAKE2B512E BLAKE2B512 BLAKE2B160E BLAKE2B160 BLAKE\
2B224E BLAKE2B224 BLAKE2B384E BLAKE2B384 BLAKE2S256E BLAKE2S256 BLAKE2S160E BLAKE2S160 BLAKE2S224E BLAKE2S224 BLAKE2SP256E BLAKE2SP256\
 BLAKE2SP224E BLAKE2SP224 SHA1E SHA1 MD5E MD5 WORM URL
remote types: git gcrypt p2p S3 bup directory rsync web bittorrent webdav adb tahoe glacier ddar hook external
operating system: linux x86_64
supported repository versions: 3 5 6
upgrade supported from repository versions: 0 1 2 3 4 5
local repository version: 5



> This was filed redundantly with [[todo/support_longer_file_extensions]]
> by I think the same person?! [[done]]
