Preview is active on file sidebar even if preview is set false on config.php

[rogerfv1]

Steps to reproduce

1. Set 'enable_previews' => false on config.php.
2. Create a 'new text file' (ex: preview.txt).
3. Click on 'Details' of that file.

Expected behaviour

The preview should not be shown on details sidebar.

Actual behaviour

The preview is showed on details sidebar.

Server configuration

Operating system: Debian 8

Web server: Apache 2.4

Database: Postgresql 9.4

PHP version: php 5.6

Nextcloud version: 12.0.3

Are you using external storage, if yes which one: no

Are you using encryption: no

Are you using an external user-backend, if yes which one: LDAP

We looked at the source code to see if we found the function that defines the preview load.
So we find a javascript function named loadPreview but it seems that this function is not considering the enable_previews set on config.php.

File: /var/www/nextcloud/apps/files/js/sidebarpreviewmanager.js

"...
loadPreview: function (model, $thumbnailDiv, $thumbnailContainer) {
if (model.get('hasPreview') === false && this.getMimeTypePreviewHandler(model.get('mimetype')) === null) {
...
"

In this case when getMimeTypePreviewHandler = 'native code' the preview is showed even you set enable_previews to false on config.php.

These are the results we got in our tests.

enable_previews (config.php)	MimeType	hasPreview	getMimeTypePreviewHandler	Preview is showed?(actual behaviour)	Preview is showed? (expected behaviour)
true	text/plain	true	native code	yes	yes
true	image/png	true	null	yes	yes
true	text/csv	false	native code	yes	yes
true	application/pdf	false	native code	yes	yes
false	text/plain	false	native code	yes	no
false	image/png	false	null	false	yes
false	text/csv	false	native code	yes	no
false	application/pdf	false	native code	yes	no

[danxuliu]

@rogerfv1 Thanks for the detailed information!

This happens in current master too, and it could be solved by providing the enable_previews value in oc_config in JSConfigHelper.php and then taking oc_config.enable_previews into account in the condition of SidebarPreviewManager.loadPreview.

For consistency it should be taken into account too in the public shared file page.

However... should enable_previews in the system configuration be used for all that, or should its effects be limited to the generation of previews in the server? In that later case it would be good to add a parameter anyway in the administration settings of the WebUI to control whether previews are shown o not. @MorrisJobke @rullzer @blizzz @nickvergessen

[blizzz]

or vice versa, loadPreviews should take enable_previews into account? after all it has an else branch, so it can continue there in case.

[fgsl]

I think that change must be implemented according to proposed by @rogerfv1 and @danxuliu.
From the point of view of sysadmin, I think that enable_previews must be used in the system configuration for all.

[fgsl]

@rullzer, according @rogerfv1, there are two reasons to disable image previews:

1. There is a note in Nextcloud documentation suggesting to disable preview for enforcing environment:
   https://docs.nextcloud.com/server/12/admin_manual/configuration_server/harden_server.html

2. For load/performance test that @rogerfv1 did (two years ago), response time of file access was better with enable_previews disabled. So, we got to serve more users with same configuration. One observation: we don't have these comparison data between times anymore.

[skjnldsv]

Fixed with #15719

[danxuliu]

If I am not mistaken the issue is hidden, but not fixed, by #15719; #15719 breaks the SidebarPreviewManager, but as soon as that is fixed I think that this issue will appear again.