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+1
I think it is very useful that the feature proposed by Go MAEDA.
I made a patch to add the feature, and attach it.
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LGTM. Setting the target version to 4.1.0.
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The patch looks good to me with one small observation, do you see any problem if we add @'text/textile' => 'textile'@ to @MIME_TYPES@? In this way, we can replace in the patch @self.filename =~ /\.(textile)$/i@ with @Redmine::MimeType.of(filename) == "text/textile"@.

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I suggest adding the following change to the patch in order to make distinguishable between the UI of Redmine and the content of the file by adding a border around the content.

<pre><code class="diff">
diff --git a/app/views/common/_markup.html.erb b/app/views/common/_markup.html.erb
index dc6c7f474..14786a87c 100644
--- a/app/views/common/_markup.html.erb
+++ b/app/views/common/_markup.html.erb
@@ -1,3 +1,3 @@
-<div class="wiki">
+<div class="filecontent wiki">
<%= Redmine::WikiFormatting.to_html(markup_text_formatting, Redmine::CodesetUtil.to_utf8_by_setting(markup_text)).html_safe %>
</div>
diff --git a/public/stylesheets/application.css b/public/stylesheets/application.css
index 5f8ade106..8c9402142 100644
--- a/public/stylesheets/application.css
+++ b/public/stylesheets/application.css
@@ -1740,6 +1740,13 @@ img {
max-width: 100%;
}

+.filecontent-container > .filecontent.wiki {
+ position: relative;
+ border: 1px solid #e2e2e2;
+ padding: 8px;
+ border-radius: 3px;
+}
+
/* Fixes for IE 11 */
@media all and (-ms-high-contrast: none), (-ms-high-contrast: active) {
select::-ms-expand {
</code></pre>

*Before:*
!{width: 918px; border: 1px solid #ccc;}.add-border-before@2x.png!

*After:*
!{width: 918px; border: 1px solid #ccc;}.add-border-after@2x.png!
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Committed the patch. Thank you all for contributing and reviewing the patch.
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Go MAEDA wrote:
> Committed the patch. [...]

What about the note Marius made in #29752#note-5?
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Mischa The Evil wrote:
> What about the note Marius made in #29752#note-5?

Oh, I forgot to handle that. Thank you for pointing it out.

Marius BALTEANU wrote:
> The patch looks good to me with one small observation, do you see any problem if we add @'text/textile' => 'textile'@ to @MIME_TYPES@? In this way, we can replace in the patch @self.filename =~ /\.(textile)$/i@ with @Redmine::MimeType.of(filename) == "text/textile"@.

I like the approach but there is only one problem that "text/textile" is not a valid media type (see https://www.iana.org/assignments/media-types/media-types.xhtml). Maybe we can use "text/x-textile" instead, what do you think?
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Go MAEDA wrote:
> Marius BALTEANU wrote:
> > The patch looks good to me with one small observation, do you see any problem if we add @'text/textile' => 'textile'@ to @MIME_TYPES@? In this way, we can replace in the patch @self.filename =~ /\.(textile)$/i@ with @Redmine::MimeType.of(filename) == "text/textile"@.
>
> I like the approach but there is only one problem that "text/textile" is not a valid media type (see https://www.iana.org/assignments/media-types/media-types.xhtml). Maybe we can use "text/x-textile" instead, what do you think?

I like your idea to use "text/x-textile".
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This patch replaces the regexp that checks '.textile' extension with @Redmine::MimeType.of@.

<pre><code class="diff">
Index: app/models/attachment.rb
===================================================================
--- app/models/attachment.rb (リビジョン 18585)
+++ app/models/attachment.rb (作業コピー)
@@ -245,7 +245,7 @@
end

def is_textile?
- self.filename =~ /\.textile$/i
+ Redmine::MimeType.of(filename) == 'text/x-textile'
end

def is_image?
Index: lib/redmine/mime_type.rb
===================================================================
--- lib/redmine/mime_type.rb (リビジョン 18585)
+++ lib/redmine/mime_type.rb (作業コピー)
@@ -35,6 +35,7 @@
'text/x-ruby' => 'rb,rbw,ruby,rake,erb',
'text/x-csh' => 'csh',
'text/x-sh' => 'sh',
+ 'text/x-textile' => 'textile',
'text/xml' => 'xml,xsd,mxml',
'text/yaml' => 'yml,yaml',
'text/csv' => 'csv',
</code></pre>
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Committed the patch #29752#note-13 in r18586.
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relates,Closed,13431,it would be nice to render the markup language in the "Repository" view
relates,Closed,16849,Render Textile and Markdown files in the repository browser
relates,New,35889,Textile and Markdown attachment rendering should support third-party formatters