Everytime I tried to upload a file to my PmWiki (popular PHP based Wiki) installation, it failed with "no file uploaded" error. Also it asks me for password thrice, once before accessing the upload page and twice after uploading.

The solution was to remove the password check on upload. As it is an intranet installation, accessed only by my team, it isn't much of a problem. To remove password check have the following line in your config.php -
$DefaultPasswords['upload'] = ";

Note: Remember to search for $DefaultPasswords first and comment the line, if present.

Bonus Tips:
1. How to increase the file upload limit in pmwiki?
Add these line to config.php -
$UploadMaxSize = 100000000;

Note: The size is in bytes; so the above is 100, 000, 000 bytes (slightly less than 100 MB).

2. How to enable file upload in the first place?
Uncomment / add this line in config.php -
$EnableUpload = 1;