Question java reflection js file format technical limitations restrict file format

Requirements : restrict users to upload file format only bmp format

But now a user suffix a non- image format and then upload the file to bmp

How can I limit this operation . java reflection technology can achieve ? some sites when uploading files will first determine whether the document it is safe is how to achieve (with time xxx upload interface antivirus company Avira ? ? ? ? ? ? ? )

idiot expert Pat
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....... good simply packaged into the virus but also to bmp uploaded .   virus package and bmp format is different. In many cases, only one file header I can tell you gave me is not what I want right file .
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you write a program , in accordance with the format to read , if not, it does not.
generally only need to determine the file header on it.
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....... good simply packaged into the virus but also to upload bmp
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....... good simply packaged into the virus but also to upload bmp .        virus package and bmp format is different. In many cases, only one file header I can tell you gave me is not what I want right file .  
................. seems like I was suddenly dark figure woman
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FormFile -----> getContextType returns the file types such as application / octet-stream