Allow user to select same file multiple times

It's common that a user may way to edit a file (e.g., to make it
smaller if given an error message), and this allows the user to
select the same file name multilple times (which ordinarily the
file element will ignore).

src/coffee/controllers/markdown_image_controller.coffee

@@ -40,6 +40,7 @@ module.exports = class MarkdownImageController extends BaseController
         if file.size > MAX_FILE_SIZE
             logger.warning "The choosen file, #{file.name}, is too large: #{file.size}"
             @errorMessage = "Choose a file less than #{MAX_FILE_SIZE / 1024}kB"
+            @_clearFile()
             return
 
         reader = @_reader = new FileReader
@@ -57,10 +58,13 @@ module.exports = class MarkdownImageController extends BaseController
                 if $image[0].naturalHeight > MAX_HEIGHT
                     logger.warning "The choosen file, #{file.name}, is too tall: #{$image[0].naturalHeight}"
                     @errorMessage = "Choose an image less than #{MAX_HEIGHT}px tall"
+                    @_clearFile()
                     return
+
                 if $image[0].naturalWidth > MAX_WIDTH
                     logger.warning "The choosen file, #{file.name}, is too wide: #{$image[0].naturalWidth}"
                     @errorMessage = "Choose an image less than #{MAX_WIDTH}px wide"
+                    @_clearFile()
                     return
 
                 index       = reader.result.indexOf ','
@@ -74,6 +78,7 @@ module.exports = class MarkdownImageController extends BaseController
                 @model.encodedData = encodedData
                 logger.info "loaded #{encodedData.length} bytes from #{file.name} as #{mimeType}"
                 @errorMessage = null
+                @_clearFile()
 
         logger.info "starting to read local file: #{file.name}"
         reader.readAsDataURL file
@@ -102,6 +107,7 @@ module.exports = class MarkdownImageController extends BaseController
         @$image          = @$('img')
         @$fileName       = @$('.fileName p')
         @$button         = @$('button')
+        @$form           = @$('form')
         @$input          = @$('input')
         @$errorContainer = @$('.error')
         @$errorMessage   = @$('.error p')
@@ -147,3 +153,8 @@ module.exports = class MarkdownImageController extends BaseController
         return _.extend super,
             'click button': 'onButtonClicked'
             'change input': 'onFileChanged'
+
+    # Private Methods ##############################################################################
+
+    _clearFile: ->
+        @$form[0].reset()