We have found a strange file which for some reason gets added to annex instead of git, although file --mime reports it to be a text file. Somehow the possible culprit (we also achieved changed in behavior via different means) is the {}

Here is the sample of a BADFILE: http://www.onerussian.com/tmp/BADFILE.txt which gets added to annex instead of git:

$> wget http://www.onerussian.com/tmp/BADFILE.txt ; cat .gitattributes; file --mime BAD
...
BADFILE.txt                      100%[=======================================================>]     289  --.-KB/s    in 0s      


* annex.backend=MD5E
* annex.largefiles=(not(mimetype=text/*))
**/.git* annex.largefiles=nothingBADFILE.txt: text/plain; charset=utf-8
add BADFILE.txt ok
(recording state in git...)

$> ls -l BADFILE.txt 
lrwxrwxrwx 1 yoh yoh 120 Apr 26 09:43 BADFILE.txt -> .git/annex/objects/xw/3W/MD5E-s289--2aae5dfcc232055ba6c06270b6c6daf0.txt/MD5E-s289--2aae5dfcc232055ba6c06270b6c6daf0.txt

so we tried to troubleshoot a bit and here is attempt with removing {} chars vs without removing which shows differing behavior:

(git)smaug:/mnt/btrfs/scrap/tmp/SIMON[master]data_BIDS
$> cat ../.gitattributes 

* annex.backend=MD5E
* annex.largefiles=(not(mimetype=text/*))
**/.git* annex.largefiles=nothing% 

$> git reset --hard; rm -f TEST.txt; sed -e 's,[{}],,g' BADFILE.txt >| TEST.txt; file --mime TEST.txt; git annex add TEST.txt
HEAD is now at f97185f badfile into git
TEST.txt: text/plain; charset=utf-8
add TEST.txt (non-large file; adding content to git repository) ok
(recording state in git...)

$> git reset --hard; rm -f TEST.txt; cat BADFILE.txt >| TEST.txt; file --mime TEST.txt; git annex add TEST.txt
HEAD is now at f97185f badfile into git
TEST.txt: text/plain; charset=utf-8
add TEST.txt ok
(recording state in git...)

$> git annex version 
git-annex version: 7.20190219+git191-g2d6a364d4-1~ndall+1
build flags: Assistant Webapp Pairing S3(multipartupload)(storageclasses) WebDAV Inotify DBus DesktopNotify TorrentParser MagicMime Feeds Testsuite
dependency versions: aws-0.20 bloomfilter-2.0.1.0 cryptonite-0.25 DAV-1.3.3 feed-1.0.0.0 ghc-8.4.4 http-client-0.5.13.1 persistent-sqlite-2.8.2 torrent-10000.1.1 uuid-1.3.13 yesod-1.6.0
key/value backends: SHA256E SHA256 SHA512E SHA512 SHA224E SHA224 SHA384E SHA384 SHA3_256E SHA3_256 SHA3_512E SHA3_512 SHA3_224E SHA3_224 SHA3_384E SHA3_384 SKEIN256E SKEIN256 SKEIN512E SKEIN512 BLAKE2B256E BLAKE2B256 BLAKE2B512E BLAKE2B512 BLAKE2B160E BLAKE2B160 BLAKE2B224E 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: 5 7
upgrade supported from repository versions: 0 1 2 3 4 5 6
local repository version: 5

$> apt-cache policy git-annex-standalone
git-annex-standalone:
  Installed: 7.20190219+git191-g2d6a364d4-1~ndall+1
  Candidate: 7.20190219+git191-g2d6a364d4-1~ndall+1
  Version table:
 *** 7.20190219+git191-g2d6a364d4-1~ndall+1 500
        500 http://neuro.debian.net/debian stretch/main amd64 Packages
        500 http://neurodebian.ovgu.de/debian stretch/main amd64 Packages
        100 /var/lib/dpkg/status

done, the magic database changing behavior is not a bug in git-annex --Joey