Oreo: associating an app with all files with a given extension?
The following looks like a promising answer ...

The file /system/lib/content-types.properties contains the following info. It looks like I can add abc=text/plain and xyz=text/plain to this file in order to get my desired behavior. I'll test this and report back.

# If you want to support more recognized mimetypes in libcore.net.MimeUtils, add them here

docm=application/vnd.ms-word.document.macroenabled.12
xlsb=application/vnd.ms-excel.sheet.binary.macroenabled.12
xlsm=application/vnd.ms-excel.sheet.macroenabled.12
ppsm=application/vnd.ms-powerpoint.slideshow.macroenabled.12
ppsx=application/vnd.openxmlformats-officedocument.presentationml.slideshow
pptm=application/vnd.ms-powerpoint.presentation.macroenabled.12

asm=text/x-asm
cc=text/x-c
conf=text/plain
cpp=text/x-c
css=text/css
c=text/x-c
cxx=text/x-c
def=text/plain
dic=text/x-c
f77=text/x-fortran
f90=text/x-fortran
for=text/x-fortran
f=text/x-fortran
hh=text/x-c
hpp=text/x-c
h=text/x-c
ics=text/calendar
ifb=text/calendar
in=text/plain
jad=text/vnd.sun.j2me.app-descriptor
java=text/x-java-source
ksh=text/plain
list=text/plain
log=text/plain
pas=text/x-pascal
pl=text/plain
properties=|
prop=text/plain
p=text/x-pascal
py=text/x-python
rc=text/plain
sgml=text/sgml
sgm=text/sgml
s=text/x-asm
vcs=text/x-vcalendar