drag'n'drop unreliable somtimes

[r10s]

sometimes, drag'n'drop is unreliable and is just not working, took me quite some time to make this reproducible.

but here is an approach on macos13.6 with deltachat1.42.0:

1. have a fresh PDF-scan in the "Notes" app
2. drag'n'drop from the "Notes" app to "Delta Chat" -> this does not work
3. in the "Notes" app, right click the PDF and select "Save as"
4. drag'n'drop from the "Notes" app to "Delta Chat" -> now same drag'n'drop works

testing with other apps (as "Apple Mail" or "Find") drag'n'drop works already at step 2. - and doing that once, this also enables things to work with "Delta Chat". "Thunderbird" has the same bug, btw

maybe the format drag'n'drop offers changes once the file is exported or otherwise used - and that "Delta Chat" only supports one format.

video showing steps 1. to 4.:

https://github.com/deltachat/deltachat-desktop/assets/9800740/76470a21-831f-41d0-b8ac-d941bcd234d6