Allowed file extensions field has bad parsing

Problem/Motivation

Create a node with a file attachment.
"Allowed file extensions" is limited to 128 characters (this is frustrating as it allows only , but not the bug)
Type in "avi, bmp, csv, doc, docx, flv, gif, jpeg, jpg, kml, mov, mpeg, mpg, mp4, pdf, png, ppt, pptx, rdf, svg, tab, tiff, txt, wmv, xls, xlsx, xml, gz, zip, tar"; it fails as it is > 128 characters
Type in "avi,bmp,csv,doc,docx,flv,gif,jpeg,jpg,kml,mov,mpeg,mpg,mp4,pdf,png,ppt,pptx,rdf,svg,tab,tiff,txt,wmv,xls,xlsx,xml,gz,zip,tar"; it succeeds as it is < 128 characters
Clicking on edit for the file attachment field produces an "exploded" list of these filetypes, which is great for readability but then requires removing of the space to successfully save the file field again

Proposed resolution

A three-character file extension takes up 5 characters currently. This allows only 25 file extensions. Increase the acceptable number of characters to 256 (50 file extensions) AND/OR avoid exploding the list of file types.

Remaining tasks

User interface changes

API changes

Original report by rmckeel