It's all about bits and bytes.
If you have Windows like me and check properties from a file, you can note that there's actually two different sizes stated for the file.
I searched for pictures from my harddrive and found one. The search window says it's 50kb.
I click properties and see Size: 49kb (50 254 bytes) and Size on disk: 64kb (65 536 bytes).
Let's try if I can attach it.
Yes I could.
The filesize for forum attachments is 50kb.
If you can attach it - ok
If you can't attach it - it's too big
* Artak kicks Kaolin arse