A partial fix has been added to CVS. This checks the file system when the project information is created or updated.
core/file_api.php -> 1.71

Longer term, an admin check does need to walk through each project and check that the system is still operational.

I had this problem on IIS. This is what I did.

Edit "core/file_api.php" file

Remark this line :
#if ( !is_readable( $p_tmp_file ) ) {
Change it to :
if ( !is_uploaded_file( $p_tmp_file ) ) {

Then go to this line and remark these 3 lines :

if ( 0 == $t_file_size ) {

#       trigger_error( ERROR_FILE_NO_UPLOAD_FAILURE, ERROR );
    # }

This is because for some reason in IIS filesize shows zero size after upload in the MySQL DB !. So we can not check for file size.. (someone here may have a better idea on how to check the size if needed).

Then go to this line and remark this line :

$c_content = db_prepare_binary_string ( fread ( fopen( $p_tmp_file, 'rb' ), $t_file_size ) ) ;

Now add the below two lines :
$tt_file_size="9000000";
$c_content = db_prepare_binary_string ( fread ( fopen( $p_tmp_file, 'rb' ), $tt_file_size ) ) ;

In the above line we define a static file size for all attachments to 9MB.
So that in the MySQL all attachemnts will show as 9MB (this is better than showing 0 MB in the Database !).

That's it.
Now you file upload must work fine in Mantis..
This is a quick hack and you can later modify to check for file size etc..

We are resolving this issue as "no change required", because it was reported against an old version of MantisBT which is no longer supported.

We recommend that you upgrade to the latest stable version [1]; if the problem persists after doing so, do not hesitate to reopen the issue.

[1] https://mantisbt.org/download.php