file format unknown is a network provider issue. In non tech terms your network gateway won't give your phone the file because it doesn't think it's a wap page.
In technical terms your wap gateway won't execpt php generated headers and is infact looking for a pure .wml/.wmls (text/vnd.wap.wml) file. PHP is giving your phone the mime type after it has agreed to recieve the file, because the gateway doesn't know what to do with the file it simply tells your phone that the file format is unknown becuase the gateway doesn't like the file extention (in this case none).
The problem can't really be solved by you. File extentions on the end of the files could help but it probably won't make a difference, however you can bypass the issue as follows.
Type into your wap browser
http://mobile.drunkit.com/view/file,IMA ... GENAME.JPG
this will then download the file to your phone. obviously swap username for your username, imagename for the name of the image, and make sure you get the file extention right (it'll most likey be .jpg, .jpeg, .gif or on rare occasions .wbmp). Make sure you get the case right, as in some cases Filename.jpg is different from filename.JPG.
A tip is to go to the file manager section of mobile.drunkit.com, and view the picture you want on your phone. Magically the text that matches what I have written above appears in the address bar on your browser/internet explorer. copy and paste this into one of those free text things like that one on lycos and text it to yourself. When you get the text simply save the link as a bookmark - load it on your phone and it will download straight away.
There is probably a quicker way. But I don't know it.
-Additional: I got the info from a book by WORX on wap, wml and wmlscript - although it was refering to asp similar would apply to PHP.
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