use client?s mimetype as fallback for uploads

www/analytics/libs/UserAgentParser/README.md

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+# UserAgentParser
+
+UserAgentParser is a php library to parse user agents,
+and extracts browser name & version and operating system.
+
+UserAgentParser is NOT designed to parse bots user agent strings; 
+UserAgentParser will only be accurate when parsing user agents 
+coming from Javascript Enabled browsers!
+
+UserAgentParser is designed for simplicity, to accurately detect the
+most used web browsers, and be regularly updated to detect new OS and browsers.
+
+Potential limitations:
+
+ * it does NOT detect sub sub versions, ie. the "5" in 1.4.5; this is a design decision to simplify the version number
+ * it does NOT detect search engine, bots, etc. user agents; it's designed to detect browsers with javascript enabled
+ * it does NOT detect nested UA strings caused by some browser add-ons
+
+Feature request:
+
+ * it could have the notion of operating system "types", ie "Windows". It currently only has "Windows XP", "Windows Vista", etc.
+
+Feedback, patches: hello@piwik.org