The same thing happens when uploading large project documentation files.

What storage are you using (DISK or DATABASE)?

If DATABASE, have you checked php.ini under the Resource Limits heading? You may be running out of execution time?

Is anything logged in the server?

Thanks thraxisp. I checked the Apache logs and found that PHP was exceeding its memory limit of 8M (yikes! for a 1.5M file upload!).

I bumped the limit to 16M and got the following error:

Database query failed. Error received from database was 0002006: MySQL server has gone away for the query: INSERT INTO mantis_bug_file_table
(bug_id, title, description, diskfile, filename, folder, filesize, file_type, date_added, content)
VALUES
(35, '', '', 'uploads/87995118ca2d96636859be99f8021d70', 'CarniglC.2004.07.26.1427.NotherCurveSetChecked.deco', 'uploads/', 1561730, 'text/plain', '2004-10-22 10:45:47', 'OCLI Persistence Engine Version 1.1\nCreated with...[rest of uploaded file]

MySQL also has similar limits for uploading entries (max_packet_size, bulk_insert_buffer).

As an aside, I've been told that MySQL is inefficient for storing large files (>64K). You may want to look at switching your attachment storage to DISK.

I didn't know about the DISK option, I much prefer this.

I've changed to that option and now all works well. Thanks for the tip thraxisp.

The powers that be may want to close this issue suggesting

$g_file_upload_method = DISK

as the fix.

Added additional warnings in admin/check.php.