Well, for completeness’ sake, here’s the official take on this:
These files are from a filesystem incompatibility somewhere on one of your linked devices (or on one of the linked devices belonging to one of the members of one of your shared folders). Some filesystems like HFS (OS X) and NTFS (Windows) are capable of assigning "extended attributes" (xattrs) to files and folders (for things like the tags you can apply in Mavericks/Yosemite).
The issue arises when files with these extended attributes are copied to a filesystem that doesn't support them (such as FAT or FAT32, which are often found on old hard drives, USB sticks, and some other external drives). These filesystems can't store extended attributes, so Dropbox puts those extended attributes into new ':com.dropbox.attributes' files, so as not to lose any data.
To test if these files are still being created, could you create a test file on a Mac, then add a tag to the file (see e.g. http://support.apple.com/kb/HT5839)? If you see another "com.dropbox.attributes" appear for this file, go to the Dropbox website, right-click on it, and choose 'View previous versions' to see which user is creating these files, and on which device.
If you do not see a new "com.dropbox.attributes" file appear for this test file, it could be the case that these files were created during a single event, such as when files were moved from a drive that doesn't support xattrs, so you can just delete them.
tl;dr: You can most likely remove them entirely without causing any problems.