It is not only for images, but also for all kinds of other documents.
For example is also the download of attached Word files no longer possible in 2.9.0.

Which PHP version do you use?
Do you store attachments in database or on file system? (check setting $g_file_upload_method for it)
What's your setting of $g_db_type?
Do you get any errors or warnings when running <YourMantisUrl>/admin/check/index.php ?

Here are your requested Infos:

MBT_admin_check.png (32,469 bytes)   

MBT_admin_check.png (32,469 bytes)   

MBT_config.png (13,570 bytes)   

MBT_config.png (13,570 bytes)   

MBT_SiteInfos.png (8,530 bytes)   

MBT_SiteInfos.png (8,530 bytes)   

FYI: The problem is most likely not the upload but the download.
My reasoning for this is that I uploaded the attachment with the 290, viewed it with 290 and then viewed the SAME entry with the 280 (parallel installation with same db settings).
-> Viewing, downloading does not work in 290
-> Viewing, downloading does work in 280

admin/check/index.php stops after getting errors.
You have to fix the errors and after that run again the script to run some more checks.
I recommend to decrease upload_max_size for it.

Do sou see any errors or warnings in your browser console or web server / PHP logs when viewing the issues with attachments?

So, I've modified the php for the two errors.. Now there are only warnings left.

For the Errors: I only got a warning in the console when I try to download the file: Invalid HTML5 DOCTYPE.
Viewing the entry only shows a warning about :visited and :link can only have different colors. (SEC7115)

MBT_admin_check_only_warnings.png (78,273 bytes)   

MBT_admin_check_only_warnings.png (78,273 bytes)   

MBT_warning_when_downloading.png (6,807 bytes)   

MBT_warning_when_downloading.png (6,807 bytes)   

Isn't the warning concerning attachments exactly what you are seeing?
Enabling the fileinfo extension might fix the issue.

Some more information concerning the change in 2.9.0
https://github.com/mantisbt/mantisbt/commit/5f6292fbabea4de238cb10cb28e3449f71fa8679

Should help http://php.net/manual/en/fileinfo.installation.php

Yes, you're right. The fileinfo extension wasn't active.
Now that I've activated it, the images are available again. :)

Thank you very much for your help :)

Thanks for confirming the solution.