[REVIEW 3-6][REVIEW 3-6-1] Fix for fdo#52451

Hi there,

I'd like

https://gerrit.libreoffice.org/gitweb?p=core.git;a=commitdiff;h=ba03f2dc817c692fa8709391ec83d8e01a0f8b28

cherry-picked to 3-6 and 3-6-1 in order to fix

https://bugs.freedesktop.org/show_bug.cgi?id=52451

Rationale:

The default extensions for saved files were determined from their 
respective filters, which in turn derived the data from their associated 
file types.  This works fine for most filters because either 1) they 
have one-to-one correspondence between the filter and the file type, or 
2) their associated file type has only one extension.

However, we need to give special treatment to the plain text file type, 
because its preferred file extension type is .txt when saving from 
Writer, and .csv when saving from Calc.  And unlike the type detection 
process during import where we can have multiple extensions for each 
file type, we can only associate one file extension for each file type 
during export.

The safest way to achieve that is to add a preferred extension property 
to the plain text filters, which motivated my fix for the bug.

Please consider backporting this to 3-6-1 if appropriate.  The change 
should only affect the plain text filters during export.

Kohei

--
Kohei Yoshida, LibreOffice hacker, Calc