fix two bugs, first is deleting tag that didn't exist was an error

second is auto was applying to all files if multiple were specified
on the command line resulting in all the mimetypes applied to all
the files...

ui/fixtures/cmd.auto.json

@@ -27,5 +27,17 @@
 	"cmd": [ "list", "z.jpg" ],
 	"stdin": "y\n",
 	"stdout_re": "mimetype:\timage/jpeg"
+},
+{
+	"title": "Test auto detection of two files",
+	"cmd": [ "auto", "z.jpg", "test.txt" ],
+	"stdin": "y\ny\n",
+	"stdout": "Set:\n\tmimetype:\timage/jpeg\n\nApply (y/N)?\nSet:\n\tmimetype:\ttext/plain; charset=us-ascii\n\nApply (y/N)?\n"
+},
+{
+	"title": "Verify modification",
+	"cmd": [ "list", "z.jpg", "test.txt" ],
+	"stdin": "y\n",
+	"stdout_re": "hashes:.*\nmimetype:\timage/jpeg.*\nsig:.*\nhashes:.*\nmimetype:\ttext/plain.*\nsig:.*\n"
 }
 ]

ui/medashare/cli.py

@@ -1030,7 +1030,10 @@ def cmd_modify(options, persona, objstr, cache):
 				try:
 					key, v = k
 				except ValueError:
-					del obj[k[0]]
+					try:
+						del obj[k[0]]
+					except KeyError:
+						pass
 				else:
 					obj[key].remove(v)
 
@@ -1372,7 +1375,7 @@ def cmd_dump(options, persona, objstr, cache):
 		print(i.encode('json'))
 
 def cmd_auto(options):
-	for i in options.files:
+	for i in options.files[:]:
 		mf = detect_from_filename(i)
 
 		primary = mf[0].split('/', 1)[0]
@@ -1388,7 +1391,8 @@ def cmd_auto(options):
 		inp = sys.stdin.readline()
 
 		if inp.strip().lower() in ('y', 'yes'):
-			options.modtagvalues = [ '+mimetype=%s' % mt ]
+			options.modtagvalues = [ '-mimetype', '+mimetype=%s' % mt ]
+			options.files = [ i ]
 			cmd_modify(options)
 
 @init_datastructs