Does your Attachment Upload mod include a resizing feature ?  For example, if a user upload a jpeg that is 1024 pixels wide, and your maximum width for attachments is 800, is the jpeg resized to 800 pixels ?   Or does an error message display ?

It shows an error when the user selects the file, allowing him to change things before submitting. 

Okay, thank you.

I use Resize Attached Images Mod, to understand that he will no longer to operate if you install your Mod?

They are not compatible. Some code would be required to make them work together.