Steps to reproduce:

I opened the preferences and scrolled to the "files & attachments" list.

Actual results:

With no filter specified, the list shows exactly three entries:
"application/x-ms-wmz type" (what's that???), "http" and "https"

No entry for pdf, no image, video or audio entries, ...

Also, filtering the list seems to behave strange:
When typing "pdf" in the filter, the list shows pdf (as expected).
Filtering for "document" results in two entries: Plain text and pdf.
"zip" or "archive" also shows the entry for "zip" (but no other archive types).
But when typing "jpg", "jpeg", "image", ... the list shows no entries at all.
For "video", "mpeg", "av1" etc. the list is also empty.
"csv" is also unknown, and "xml" shows "Microsoft word (open xml)" instead of plain xml.

Expected results:

With no filter specified, the list should show all entries, not only three.

Entering filter strings shows that there are definitely more entries than the three entries shown without filtering (e.g. pdf, plain text and zip), so the list without filtering is incomplete.

But many document types seem to be really missing, e.g. all image, video and audio types, html, xml and csv, ...

IIRC they are added to the list once you first use one of the type...

No, definitely not.

I've opened many pdf, microsoft office and jpeg attachments in my mails,
and none of these types shows up in the preferences without a filter.
Some (e.g. Microsoft Excel) do not even show up when filtering for them.

lets start with what operating system you are observing this rather odd behaviour on.

I have a daily version on windows 10 which I have to the best of my knowledge never opened an attachment in. It contains only one entry. PDF set to preview in Daily.

application/x-ms-wmz appears to be a windows media player skin file. https://help.clouduss.com/ws-knowledge-base/mime-categories-and-types
But it is actually an unregistered type. https://www.iana.org/assignments/media-types/media-types.xhtml#application and as such would not be any sort of built in media foprmat.

Operating system is linux,
and this is a thunderbird installation which has been in heavy use for years and is now 91.3.2.

I re-checked again.

With no filter, the list shows application/x-ms-wmz: "always ask", html (which I've set explicitly due to bug 1740759 ): "always ask", http: "Use Web browser (default)", and https: Right column Empty?!

First problem:
Some entries (like "pdf" or plain text) are shown only when explicitely filtering for them, not with an empty filter.
With no filter, all entries should be shown, and filtering should reduce the number of entries shown.

Second problem:
Many types are not shown at all (like all graphics and multimedia formats), although I've definitely used and opened them in the past,
and if I remember correctly, for some of them I've also set non-standard tools.

Problem still exists in TB 102.x:

1.) The list of file types and applications shown in "files & attachments" ist very incomplete:
Without any filter, most file types are not shown, not even pdf or plain text (not even those having custom settings here).
The list has just 5 entries: application/x-ms-wmz: always ask, GIF image: use geequi (default), HTML document: always ask,
http: use web browser (default), https: <empty right column!>
Expected behaviour: Without a filter, all entries must be shown in the list.

2.) Some misssing entries appear when filtering for them (e.g. PDF),
but many are either not shown even with a filter,
or I'm unable to guess the correct filter string matching them.

Do we have this covered under another bug report? If not, what is next step?

I don't really know. This is basically functionality we get from core.

If "core" means Firefox:
The same application list in the Firefox settings behaves very different:

• Without any filter, it shows many entries more than thunderbird, most likely it shows all entries firefox knows about (as expected).
• Typing a filter always reduces the numer of entries shown,
  it never causes any new items to appear in the list which have not been shown in the unfiltered list.