File name extensions are corrupted.
This is a bug in certain versions of Mac OS X unrelated to Mail Attachments Iconizer. When there are non-ASCII characters (e.g. ä, é or ®) in the file name Mail often corrupts the file name extension (e.g. .pdf, .jpg or .doc) which is often necessary to determine the file type.
Workaround: The bug seems to be fixed in the recent Mac OS X updates (as of version 10.5.4). Additionaly it is more safe in general to only use ASCII characters in attachment file names, and to avoid periods (full stops) except before the file name extension. (ASCII characters include unaccented letters of the Latin alphabet, digits, basic punctuation and spaces.)