Macintosh files consists of two parts -- a resource fork and a data fork. When you download files on a MS-DOS computer, you loose resource fork since MS-DOS/Windows only understands how to handle the data portion of the file. However since Macintosh files are upload using the MacBinary file format, you can occasionally fix the file (this does not always work).
The MacBinary format places a 128 byte header on the uploaded file. This header, if not removed from the file, makes the file unusable on the Macintosh. There are some public domain and commercial application programs for the Macintosh that can remove the 128 byte MacBinary header to make the file usable again. Sometimes after transferring the file from one platform the file is corrupted and there is nothing you can do to fix the file other than download the file again using a Macintosh computer.