Q:

Why does the Mobile Safari Browser on iOS not allow file uploads?

As already known, it's not possible for iOS users to select image files to upload from Safari on iPhone, because the browse button to display the "select file"- dialog is disabled. It works fine on Android, but not on iPhone... 
What is the particular reason for that issue? 
I heard that the browse button is disabled because there isn't a file browser on the iPhone. Someone other said that Safari just disabled root access. Is there any reliable source which explains the issue? (I need it for my thesis.)

A:

iOS apps can only read data in their own directory and thus only Safari would be able to use these files - so how could you create them and put them in Safari's directories?
From Apple's Filesystem basics development documentation

An iOS app’s interactions with the file system are limited mostly to
  the directories inside the app’s sandbox. During installation of a new
  app, the installer code creates a home directory for the app, places
  the app in that directory, and creates several other key directories.
  These directories constitute the app’s primary view of the file
  system. Figure 1-1 shows a representation of the sandbox for an app.
Figure 1-1  Each iOS app operates within its own sandbox
Because it is in a sandbox, an app is generally prohibited from
  accessing or creating files in directories outside of its home
  directory. One exception to this rule occurs when an app uses public
  system interfaces to access things such as the user’s contacts or
  music. In those cases, the system frameworks handle any file-related
  operations needed to read from or modify the appropriate data store

A:

iOS 6 fixed this problem by granting one exception to the sand boxing of apps from each-other's data. Upload buttons may now ask about taking/browsing for a picture or video clip to upload. 
Other than by exception, there still is no full filesystem access on iOS 6 and lower. 

A:

From all of my research I cannot find a descriptive answers as to why this feature has been disabled in mobile safari. I don't know if you are interested but if you have your iOS device jailbroken there is a tweak called "Safari Upload Enabler" which gives mobile safari the ability to upload files. You can buy it on cydia for $1.99

