Why are "com.dropbox.attr" attributes added to my Dropbox?

Hey it9, thanks for reaching out to our Community.

Let me see if I can clarify this situation.

Basically, these files are hidden additional information that are part of the file but can't normally be seen.

These files may be generated by either Dropbox or a computer to retain this information in situations where the file system on the computer does not support the extended attributes.

The supported file systems are NTFS for Windows, HFS for Mac and on Linux a file system with xattrs support.

If a file with unsupported attributes is added to a computer's Dropbox folder from a networked drive with a different file system than that of your main drive's file system, the OS on your computer may generate these extended attribute files. 

Could that be the case?

You can read more about this here.

Let me know if you have more questions.