Upload problem when mixing images and PDF file

[eluser]LuckyFella73[/eluser]
hi there,

I scripted a small backend where the user can upload 2 images
and a pdf file. Each file is checked/uploaded by a seperated
callback function.

The problem: I can upload 1 or 2 images OR the PDF file but not
all together. When selecting 1 or 2 images AND a PDF file
I get the error message "The filetype you are attempting to upload is not allowed".
In my tests I allways selected the same file (regarding to each input field).

Any ideas what could cause this problem?