I recently set up a C5 website for a client, but had a lot of trouble with the uploader. I tried adding an .htaccess file to raise the filesize limits, but GoDaddy didn't allow the lines I wrote in. I added php5.ini and php.ini files to raise the limits, but even with exorbitantly high limits, it didn't work. I was getting frustrated. Nothing would upload. I thought it was a C5 issue.
You may be familiar with the message: "An unknown error occurred while uploading the file. Please check that file uploads are enabled, and that your file does not exceed the size of the post_max_size or upload_max_filesize variables."
It's a PHP5 issue. Here's what you need to know:
In your php5.ini file, you do need to make sure your filesize limits are high enough and that file_uploads is set to "on", but much more importantly, you need to ensure that upload_tmp_dir is set to "/tmp/". I don't know if it's a GoDaddy thing (probably, I've had many bad experiences with them) or a PHP5 thing, but I didn't know that it no longer has a default value. It usually does.
Anyway, that's it! It's the most trivial thing. It took hours of digging to find the answer buried in a forum about PHP. Hopefully I save some people a bit of trouble.