Importing files: Some file types (.tex, .odt) only show up under "All files"

[Tobias Hilbricht]

Dear readers of this list,
I am using LyX 2.3.4.2 on Ubuntu 18.04.4 and have the following issue:
When clicking File -> Import File -> LaTeX (plain), a window pops up
allowing to choose a LaTeX.tex file. However, when navigating to a
folder which contains LaTeX.tex files and the menu just above the
button "Open" says "LaTeX", no file at all shows up. When changing the
menu button to "All Files" all files are shown. The same is true for
OpenDocument.odt files - they do not show up specifically when the menu
says "OpenDocument", but only among many others, which lengthens the
process of file choosing. 

By contrast, when trying to import HTML, the menu just above the "Open"
button reads "HTML", and all HTML-files in the folder are shown; it
works as expected for OpenDocument spreadsheets (ods), too, and for MS-
Word Office-Open XML (docx).

Is this a local malconfiguration - if so, how can I reconfigure my
system properly - or is this a bug?

Thanks for helpful hints
Tobias Hilbricht