image upload -> mime types??

i've created an application where users can upload images through the form upload, and to ensure that they are sending me an image, i take a look at the type of the file (ie. $uploadedfile_type), which usually returns something like image/x-png however.. not always.. so secondly, i check for a file extension using $uploadedfile_name, but if they've loaded it from a mac.. i can't be sure there will be a filename.. so, those things both failing in some cases.. is there any other way of checking the filetype of a file? i'm concerned that some could upload malicious content and run it (although the execute flag is turned off, AND the filename is difficult to get.. ) and would like to reduce the possiblity.. any suggestions? -- matthew knight - online developer matthew@deepend.co.uk