I ran into a similar problem on Ubuntu 17.10 that I think is related to
the apparmor profile, because it appeared recently, but has gone away
with the newest release.  I have an ODT file (in my home folder) that
has no ".odt" at the end of the filename.

It has never had problems in the past (Nautilus recognizes the file type
from the contents, and launches LibreOffice, which would open the file
successfully), but suddenly it started giving me an access denied
message, which went away if I added ".odt" to the name.  Now with the
newest release, it works again without the ".odt".

Confined apps seem like a good idea, but seeing the variety of different
problems that this change has caused (and will probably cause in the
future as more apps apply confinement) makes me a bit concerned about
the overall confinement strategy in general, not just for LibreOffice.

It seems like trying to make a universal definition of what file names and 
locations are appropriate for an app to access is an impossible task.  The 
problem is that in a broad conceptual sense, there are sort of three classes of 
files: 
System Files: need to be protected from untrusted users and untrusted apps.  OS 
determines names and directory structure
App Files: owning app needs has full automatic access, and chooses names and 
sub-directory structure.
User Files: user has full access, and chooses names and sub-directory 
structure. 

It seems to me that a better approach would be to give each app one
specific folder that it has completely free access to read and write
"app-owned" files, like settings, saved games (maybe
"~/.local/share/appname" or "~/Settings/Apps/appname"), and then have
specific "Trusted File Pickers" (for example, Nautilus, or a system-
provided Open/Save window) that could temporarily give the app
permission to access a specific file selected by the user.

I don't know if apparmor is capable of doing this sort of thing, but it
seems like a better general approach to app confinement.  I don't want
an untrusted application to freely read or write everything in my home
folder, potentially stealing personal information, or
deleting/encrypting my documents.  I also don't want an app to be
prevented from accessing a file that I explicitly try to open with it,
just because of where it's stored, or how the filename is formatted.

The one downside I can see for a system like I proposed is that it would
make it very hard for confined applications to provide their own custom
Open/Save windows, but I think that would be a reasonable trade-off for
having more security.

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https://bugs.launchpad.net/bugs/1751005

Title:
  libreoffice cannot open a document not within $HOME

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