I know I can define file type using in appinfo.plist such as zip, pdf etc
Is it possible define any type with special extension, such as '.mydatafiletype' or '.datafiletype'
Welcome any comment
Yes, you can. The documentation clearly states so.
 
    
    Sure you can.
 Generally, you need to set UTExportedTypeDeclarations property in info.plist file.
The post here: How do I associate file types with an iPhone application? 
Gives you a sample to do it. 
Hope this helps.
 
    
    