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Sorry to hear you’re facing problems 
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In order to help you as quickly as possible, before clicking Create Topic please provide as much of the below as you can. Feel free to use a pastebin service for logs, otherwise either indent short log examples with four spaces:
example
Or for longer, use three backticks above and below the code snippet:
longer
example
here
Some or all of the below information will be requested if it isn’t supplied; for fastest response please provide as much as you can 
Nextcloud version (eg, 12.0.2): 15
Operating system and version (eg, Ubuntu 17.04): docker: fpm-alpine
The issue you are facing:
The integrity checker fails after the upgrade on the file core/js/mimetypelist.js. Looked on forum for resolution and saw that the closest one suggests to turn off integrity check, and that is frowned upon, but I could find no actual resolution to the issue.
Is this the first time you’ve seen this error? (Y/N): Y
Steps to replicate it:
- Upgrade to version 15
- Go to overview and see result of integrity check.
The output of your Nextcloud log in Admin > Logging:
Technical information
=====================
The following list covers which files have failed the integrity check. Please read
the previous linked documentation to learn more about the errors and how to fix
them.
Results
=======
- core
- INVALID_HASH
- core/js/mimetypelist.js
Raw output
==========
Array
(
[core] => Array
(
[INVALID_HASH] => Array
(
[core/js/mimetypelist.js] => Array
(
[expected] => dc48de7ad4baa030c5e563350c9a80b274bad783f6f5adbf1595ecef6c6a32e52890a24cb26cddb0aa20193ba52c001150c68d8bfb567f0aed566f4029a190a3
[current] => 53613657e01ad07fb878200f174a72a8ac062f6f3d52624034c85157a2bd7517b6b79b18ddad5b312319e7d9bec6b1d409c8ec9098480a356777e0dcee2f39f8
)
)
)
)
The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
<?php
$CONFIG = array (
'memcache.local' => '\\OC\\Memcache\\APCu',
'apps_paths' =>
array (
0 =>
array (
'path' => '/var/www/html/apps',
'url' => '/apps',
'writable' => false,
),
1 =>
array (
'path' => '/var/www/html/custom_apps',
'url' => '/custom_apps',
'writable' => true,
),
),
'memcache.locking' => '\\OC\\Memcache\\Redis',
'redis' =>
array (
'host' => 'redis',
'port' => 6379,
),
'instanceid' => '*******',
'passwordsalt' => '******',
'secret' => '*************',
'trusted_domains' =>
array (
0 => '**************',
),
'datadirectory' => '/var/www/html/data',
'overwrite.cli.url' => '************',
'dbtype' => 'mysql',
'version' => '15.0.0.10',
'dbname' => '***********',
'dbhost' => '************',
'dbport' => '',
'dbtableprefix' => 'oc_',
'dbuser' => '********',
'dbpassword' => '***********',
'installed' => true,
'mail_smtpmode' => 'smtp',
'mail_smtpauthtype' => 'PLAIN',
'mail_from_address' => '*********',
'mail_domain' => '*******',
'mail_smtphost' => '******',
'mail_smtpport' => '25',
'maintenance' => false,
'updater.release.channel' => 'production',
'loglevel' => 2,
'theme' => '',
);