Add detect function in _xls. Update yaml, csv, and tsv detection functions to catch other errors when faced with invalid input.

tablib/formats/_csv.py

@@ -51,5 +51,5 @@ def detect(stream):
     try:
         csv.Sniffer().sniff(stream)
         return True
-    except csv.Error:
-        return False
+    except (csv.Error, TypeError):
+        return False

tablib/formats/_tsv.py

@@ -55,5 +55,5 @@ def detect(stream):
     try:
         csv.Sniffer().sniff(stream, delimiters='\t')
         return True
-    except csv.Error:
+    except (csv.Error, TypeError):
         return False

tablib/formats/_xls.py

@@ -6,6 +6,8 @@
 import sys
 
 from tablib.compat import BytesIO, xlwt
+from tablib.packages import xlrd
+from tablib.packages.xlrd.biffh import XLRDError
 import tablib
 
 title = 'xls'
@@ -16,6 +18,24 @@ wrap = xlwt.easyxf("alignment: wrap on")
 bold = xlwt.easyxf("font: bold on")
 
 
+def detect(stream):
+    """Returns True if given stream is a readable excel file."""
+    try:
+        xlrd.open_workbook(file_contents=stream)
+        return True
+    except (TypeError, XLRDError):
+        pass 
+    try:
+        xlrd.open_workbook(file_contents=stream.read())
+        return True
+    except (AttributeError, XLRDError):
+        pass
+    try:
+        xlrd.open_workbook(filename=stream)
+        return True
+    except:
+        return False
+
 
 def export_set(dataset):
     """Returns XLS representation of Dataset."""

tablib/formats/_yaml.py

@@ -60,5 +60,5 @@ def detect(stream):
             return True
         else:
             return False
-    except yaml.parser.ParserError:
-        return False
+    except (yaml.parser.ParserError, yaml.reader.ReaderError):
+        return False