Unrestricted php file upload fix (#681)

https://huntr.dev/bounties/d7453360-baca-4e56-985f-481275fa38db/

app/Rules/Base64Mime.php

@@ -0,0 +1,85 @@
+<?php
+
+namespace Crater\Rules;
+
+use Illuminate\Contracts\Validation\Rule;
+
+class Base64Mime implements Rule
+{
+    private $attribute;
+    private $extensions;
+
+    /**
+     * Create a new rule instance.
+     *
+     * @return void
+     */
+    public function __construct(array $extensions)
+    {
+        $this->extensions = $extensions;
+    }
+
+    /**
+     * Determine if the validation rule passes.
+     *
+     * @param  string  $attribute
+     * @param  mixed  $value
+     * @return bool
+     */
+    public function passes($attribute, $value)
+    {
+        $this->attribute = $attribute;
+
+        try {
+            $data = json_decode($value)->data;
+        } catch (\Exception $e) {
+            return False;
+        }
+
+        $pattern = '/^data:\w+\/[\w\+]+;base64,[\w\+\=\/]+$/';
+
+        if(!preg_match($pattern, $data)) {
+            return False;
+        }
+
+        $data = explode(',', $data);
+
+        if(!isset($data[1]) || empty($data[1])) {
+            return False;
+        }
+
+        try {
+            $data = base64_decode($data[1]);
+            $f = finfo_open();
+            $result = finfo_buffer($f, $data, FILEINFO_EXTENSION);
+
+            if($result === '???')
+                return False;
+
+            if(strpos($result, '/')) {
+                foreach(explode('/', $result) as $ext) {
+                    if(in_array($ext, $this->extensions))
+                        return True;
+                }
+            } else {
+                if(in_array($result, $this->extensions))
+                    return True;
+            }
+        } catch (\Exception $e) {
+            return False;
+        }
+        
+        return False;
+
+    }
+
+    /**
+     * Get the validation error message.
+     *
+     * @return string
+     */
+    public function message()
+    {
+        return 'The ' . $this->attribute . ' must be a json with file of type: ' . implode(', ', $this->extensions) . ' encoded in base64.';
+    }
+}