Web-based file upload -- who grants/denies permissions to whom?

Hello,

After all this time, Linux file permissions still confuse me at times.

I have a Python web-based application with an file (images) upload module. The application is running on two remote servers and a local server on my development network.

The upload module works just fine when I'm uploading to my remote servers. But when I try to upload to my development server, I get a permission error.

What I don't understand is who is granting/denying permission to whom?

Let's say:

-- the file to be downloaded is owned by dddd and group dddd
-- others in dddd have read permission but not write permission
-- the directory the target directory for download is tttt and group tttt

Question 1: Should I be looking at my Python upload code or my permission scheme to allow download to my development server?

Question 2: If Python upload code, what should the code look like?

Question 3: If permission scheme, how should I change it to permit download to my development server?

Question 4: From a security perspective, what is best practice for a) assigning file and directory permissions in web-based applications; b) crafting the upload module?

Many thanks,

Lloyd