I found the solution; I had opcache enabled in PHP; as soon as I disabled then the attachment feature worked correctly

my opcache settings were these below

[opcache]
zend_extension==C:\PHP\7.1.3\ext\php_opcache.dll
opcache.enable=1
opcache.memory_consumption=128
opcache.interned_strings_buffer=8
opcache.max_accelerated_files=4000
opcache.revalidate_freq=60
opcache.fast_shutdown=1
opcache.enable_cli=1

I think that I had that enabled for word press

Thanks for telling the solution.

no problem; it would be nice to know if I can use opcache but with different settings

I assume this is a bug in Opcache.
Using latest 7.1.x version (7.1.12 at the moment) might fix the issue as some Opcache bugs have been fixed since 7.1.3.

OK I'll try, I am using PHP 7.1.5. I will download the latest and re-enable opcache and etst it. I will keep you posted

I updated my working v2.8.0 to v2.9.0, and now I have the very same problem.

As far I can see, I don't have opcache enabled.

I have the very same problem.

Does it mean you get the same error message ?

<yourURL> is currently unable to handle this request HTTP ERROR 500

If not, open a new issue for it.
Before that, check if you get any errors or warnings when running <YourMantisUrl>/admin/check/index.php and fix them.

The very same problem.

Using Firefox i have a blank screen.
Using Edge I have error HTTP ERROR 500.

I also updated to latest PHP 7.1.12, but nothing changes.

my previosly working MBT cfg params are:

# 5.19 File Uploads

$g_allow_file_upload    = ON;

$g_file_upload_method   = DISK;

$g_absolute_path_default_upload_folder = 'e:/web/uploads/mantisbt/'; # used with DISK, must contain trailing \ or /.

$g_max_file_size        = 2097152;  # in bytes

$g_allowed_files        = 'zip,7z,jpg,png';     # extensions comma separated, e.g. 'php,html,java,exe,pl'

Did you

check if you get any errors or warnings when running <YourMantisUrl>/admin/check/index.php and fix them.

I ran it and it reported that I miss '\' at the end of $g_absolute_path_default_upload_folder.

I changed all present '/' to '\', and now it seems working fine.

Thank you

wait... I verified a v2.8.0 link...

same bug on v2.9.0 still has the problem

digging around, I enabled 'php_fileinfo.dll' extension into my php.ini, and it seems to solve the problem.

I wonder what is changing about between the 2 versions.

digging around, I enabled 'php_fileinfo.dll'

Seems you did not follow my instructions

Before that, check if you get any errors or warnings when running <YourMantisUrl>/admin/check/index.php and fix them.

It would have told you the reason for the problem.

So your issue is the same like 0023707

I wonder what is changing about between the 2 versions.

https://github.com/mantisbt/mantisbt/commit/5f6292fbabea4de238cb10cb28e3449f71fa8679

[../check/index.php]

I compared the report of the two versions, and I have the same warnings (excluding some security items to verify).

In both versions, I had the following:

Attachments

Fileinfo extension is available for determining file MIME types

Web clients may struggle to download files without knowing the MIME type of each attachment.

This put me in the right direction.

... later I have discovered, that using this extension (under Windows) is causing failure on uploading files.

Now I rolled back to v2.8.1 that is working fine.