The help text and feedback messages regarding file upload size all claim the maximum size is set for PHP with PHP's upload_max_filesize parameter, but in fact, if upload_max_filesize is more than half of the PHP post_max_size parameter, file_upload_max_size() silently limits upload_max_filesize to only half of post_max_size.
I have 2 problems with this:
1. The only way to discover this is to read the code in file.inc -- usually only after wasting all kinds of time looking in all the places Drupal tells you to look (e.g. php.ini, settings.php, .htaccess, etc.).
2. The programmer who wrote this was just being lazy, as is evidence by the preceding comment which says:
// sanity check- a single upload should not be more than 50% the size limit of the total post
That's a lame sanity check. I should be able to upload a posting where only a few bytes are content and the rest is attached file without being limited by half the max size for the file.
This "mis-design" is in 4.7 and HEAD.