I cannot upload ANY images other than PNG. If I change the allowed file formats to "" or nothing, which should allow ANY file format, I then am unable to upload ANY format.
I just tried this with the following:

Base Drupal Install (Drupal 6.26)
Contrib Modules enabled:
Project: admin_menu 6.x-1.8
Administration menu (admin_menu)
Project: cck 6.x-2.9
Content (content)
Project: drupal 6.26
Color (color)
Comment (comment)
Database logging (dblog)
Help (help)
Menu (menu)
Taxonomy (taxonomy)
Update status (update)
Project: enabled_modules 6.x-1.0
Enabled modules (enabled_modules)
Project: filefield 6.x-3.10
FileField (filefield)
Project: imageapi 6.x-1.10
ImageAPI (imageapi)
ImageAPI GD2 (imageapi_gd)
Project: imagecache 6.x-2.0-beta12
ImageCache (imagecache)
Project: imagefield 6.x-3.10
ImageField (imagefield)
Project: jquery_update 6.x-1.1
jQuery Update (jquery_update)
Project: libraries 6.x-1.0
Libraries (libraries)
Project: node_gallery 6.x-3.2
Node Gallery (node_gallery)
Project: plupload 6.x-1.x-dev
Plupload integration module (plupload)
Project: views 6.x-2.16
Views (views)

Additional:
Plupload v1.5.4 installed in (/sites/all/libraries/plupload)