Not able to open card attachments

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Describe the bug
We attached files (txt and pdf) to cards and sometimes it is not possible to open the attachment.
After making an upload of pdf-Files the files are displayed in the cards details attachment as PDF-Files.
Leaving NextCloud and opening it some times later again the files may not be displayed as PDF-Files. Trying to open this files shows a JSON-File, e.g. see attached file "Attachment27.json.txt".
Attachment27.json.txt

To Reproduce
This is a random problem. Sometimes it happens, sometimes it works without any trouble.

I tried different file names:

• AttachmentTest1_working.pdf
• AttachmentTest2 Blank.pdf
• AttachmentTest3_UmlautÄ.pdf
• AttachmentTest4_Umlautö.pdf
• AttachmentTest5_SZß.pdf

but the problem is independent of the file name.

The problem is independent of the creator of the card / attacher of the file.
If the problem is "active" an upload of the same file shows it as "undefined content" and as pdf, see Screenshot

Expected behavior
Attached files shall be displayable everytime.

Screenshots

Client details:
Chromium 72.0.3626.121 (Offizieller Build) / Firefox 83.0 (32-Bit)
Raspbian 9.13 (32-Bit) /Windows
Device: Desktop

(I'm not the administrator of the installation!)
DECK 1.1.2
NextCloud 19.0.4

Login as admin user into your Nextcloud and access
http://example.com/index.php/settings/integrity/failed
paste the results here.

If you have access to your command line run e.g.:
sudo -u www-data php occ app:list
from within your Nextcloud installation folder

If you have access to your command line run e.g.:
sudo -u www-data php occ config:list system
from within your Nextcloud installation folder

or

Insert your config.php content here
Make sure to remove all sensitive content such as passwords. (e.g. database password, passwordsalt, secret, smtp password, …)

Insert your Nextcloud log here

Insert your browser log here, this could for example include:

a) The javascript console log
b) The network log
c) ...

Start by reproducing the intermittent attachment behavior with the listed PDF and TXT examples, then inspect the browser and network logs requested in the issue and compare the attached Attachment27.json.txt response. The issue is complete when card attachments remain correctly identified and can be opened after leaving and revisiting Nextcloud.