When making or editing a field of the type field it is possible to set the file extensions. Some are PDF-safe and some are not. PDF-safe means that the file can be converted on the server in a pdf.

The following options are available:

 
Choice

File extension

any files
compressed files (zip) zip
documents (rtf/doc/docx) doc, docx, or rtf
documents (rtf/doc/docx) or pdf doc;docx;rtf;pdf
images only (PDF-SAFE) jpg, gif or png
images or pdf (PDF-SAFE) jpg;gif;png;pdf
pdf only (PDF-SAFE) pdf