File Paths are slightly different in the two platforms.
Rather than post a short explanation, this Google has good explanations in the top five links
With number one being the Filemaker Inc Help Page for FilePaths
If you have set this up like I think you have, you are absolutely correct that the problem lies in the differences between Mac and Windows systems.
I believe you have container fields where you store references to your "uploaded" files rather than a physical copy of the actual file. This works for all users on your system only if the file path from their computer to the shared directory where this file is stored is identical for all users. Since windows systems typically Start such a file path with a Drive letter like D: and mac systems do not, you likely do not have identical paths to this shared directory. (The directory must also be mounted in order for filemaker to access the file also.)
One solution is to define a calculation field that returns a container type. You can construct a calculation that extracts the filepath from the container where you inserted the file--specifying store by reference and then produces a file path that starts with the text appropriate to the current user's operating system and then appends the extracted filepath. The resulting container field then works from both platforms as long as the shared directory is correctly mapped and mounted.