This is happening since a few months ago but I haven't reported it
because I tought it was going to be fixed shortly (shame on me).

What happens is that whenever I want to open any PDF file by
double-clicking on it I got the following error:

==============
Cannot open nRF2401A.pdf

The filename "nRF2401A.pdf" indicates that this file is of type "pdf
document". The contents of the file indicate that the file is of type
"PDF document". If you open this file, the file might present a security
risk to your system.

Do not open the file unless you created the file yourself, or received
the file from a trusted source. To open the file, rename the file to the
correct extension for "PDF document", then open the file normally.
Alternatively, use the Open With menu to choose a specific application
for the file.
===============

In this case nRF2401A.pdf is some PDF document, but the same happens
with any PDF document.

As you can see it refuses to open the file because the file-type and
content-type differ only in letter case, even when the type is the same.

I guess nautilus will refuse to open any other file which reports a
letter case mistmatch between file-type and content-type.