diff --git a/src/main/scala/app/WikiController.scala b/src/main/scala/app/WikiController.scala
index 2d7a588..3bffdb3 100644
--- a/src/main/scala/app/WikiController.scala
+++ b/src/main/scala/app/WikiController.scala
@@ -1,13 +1,14 @@
 package app
 
 import service._
-import util.{CollaboratorsAuthenticator, ReferrerAuthenticator, JGitUtil, StringUtil}
+import util._
 import util.Directory._
 import util.ControlUtil._
 import jp.sf.amateras.scalatra.forms._
 import org.eclipse.jgit.api.Git
-import org.eclipse.jgit.api.errors.PatchApplyException
 import org.scalatra.FlashMapSupport
+import service.WikiService.WikiPageInfo
+import scala.Some
 
 class WikiController extends WikiControllerBase 
   with WikiService with RepositoryService with AccountService with ActivityService
@@ -17,20 +18,22 @@
   self: WikiService with RepositoryService with ActivityService
     with CollaboratorsAuthenticator with ReferrerAuthenticator =>
 
-  case class WikiPageEditForm(pageName: String, content: String, message: Option[String], currentPageName: String)
+  case class WikiPageEditForm(pageName: String, content: String, message: Option[String], currentPageName: String, id: String)
   
   val newForm = mapping(
-    "pageName"        -> trim(label("Page name"          , text(required, maxlength(40), pagename, unique))),
-    "content"         -> trim(label("Content"            , text(required))),
-    "message"         -> trim(label("Message"            , optional(text()))),
-    "currentPageName" -> trim(label("Current page name"  , text()))
+    "pageName"        -> trim(label("Page name"         , text(required, maxlength(40), pagename, unique))),
+    "content"         -> trim(label("Content"           , text(required, conflictForNew))),
+    "message"         -> trim(label("Message"           , optional(text()))),
+    "currentPageName" -> trim(label("Current page name" , text())),
+    "id"              -> trim(label("Latest commit id"  , text()))
   )(WikiPageEditForm.apply)
   
   val editForm = mapping(
-    "pageName"        -> trim(label("Page name"          , text(required, maxlength(40), pagename))),
-    "content"         -> trim(label("Content"            , text(required))),
-    "message"         -> trim(label("Message"            , optional(text()))),
-    "currentPageName" -> trim(label("Current page name"  , text(required)))
+    "pageName"        -> trim(label("Page name"         , text(required, maxlength(40), pagename))),
+    "content"         -> trim(label("Content"           , text(required, conflictForEdit))),
+    "message"         -> trim(label("Message"           , optional(text()))),
+    "currentPageName" -> trim(label("Current page name" , text(required))),
+    "id"              -> trim(label("Latest commit id"  , text(required)))
   )(WikiPageEditForm.apply)
   
   get("/:owner/:repository/wiki")(referrersOnly { repository =>
@@ -60,45 +63,43 @@
   
   get("/:owner/:repository/wiki/:page/_compare/:commitId")(referrersOnly { repository =>
     val pageName = StringUtil.urlDecode(params("page"))
+    val Array(from, to) = params("commitId").split("\\.\\.\\.")
 
-    defining(params("commitId").split("\\.\\.\\.")){ case Array(from, to) =>
-      using(Git.open(getWikiRepositoryDir(repository.owner, repository.name))){ git =>
-        wiki.html.compare(Some(pageName), from, to, JGitUtil.getDiffs(git, from, to, true), repository,
-          hasWritePermission(repository.owner, repository.name, context.loginAccount), flash.get("info"))
-      }
+    using(Git.open(getWikiRepositoryDir(repository.owner, repository.name))){ git =>
+      wiki.html.compare(Some(pageName), from, to, JGitUtil.getDiffs(git, from, to, true), repository,
+        hasWritePermission(repository.owner, repository.name, context.loginAccount), flash.get("info"))
     }
   })
   
   get("/:owner/:repository/wiki/_compare/:commitId")(referrersOnly { repository =>
-    defining(params("commitId").split("\\.\\.\\.")){ case Array(from, to) =>
-      using(Git.open(getWikiRepositoryDir(repository.owner, repository.name))){ git =>
-        wiki.html.compare(None, from, to, JGitUtil.getDiffs(git, from, to, true), repository,
-          hasWritePermission(repository.owner, repository.name, context.loginAccount), flash.get("info"))
-      }
+    val Array(from, to) = params("commitId").split("\\.\\.\\.")
+
+    using(Git.open(getWikiRepositoryDir(repository.owner, repository.name))){ git =>
+      wiki.html.compare(None, from, to, JGitUtil.getDiffs(git, from, to, true), repository,
+        hasWritePermission(repository.owner, repository.name, context.loginAccount), flash.get("info"))
     }
   })
 
   get("/:owner/:repository/wiki/:page/_revert/:commitId")(collaboratorsOnly { repository =>
     val pageName = StringUtil.urlDecode(params("page"))
+    val Array(from, to) = params("commitId").split("\\.\\.\\.")
 
-    defining(params("commitId").split("\\.\\.\\.")){ case Array(from, to) =>
-      if(revertWikiPage(repository.owner, repository.name, from, to, context.loginAccount.get, Some(pageName))){
-        redirect(s"/${repository.owner}/${repository.name}/wiki/${StringUtil.urlEncode(pageName)}")
-      } else {
-        flash += "info" -> "This patch was not able to be reversed."
-        redirect(s"/${repository.owner}/${repository.name}/wiki/${StringUtil.urlEncode(pageName)}/_compare/${from}...${to}")
-      }
+    if(revertWikiPage(repository.owner, repository.name, from, to, context.loginAccount.get, Some(pageName))){
+      redirect(s"/${repository.owner}/${repository.name}/wiki/${StringUtil.urlEncode(pageName)}")
+    } else {
+      flash += "info" -> "This patch was not able to be reversed."
+      redirect(s"/${repository.owner}/${repository.name}/wiki/${StringUtil.urlEncode(pageName)}/_compare/${from}...${to}")
     }
   })
 
   get("/:owner/:repository/wiki/_revert/:commitId")(collaboratorsOnly { repository =>
-    defining(params("commitId").split("\\.\\.\\.")){ case Array(from, to) =>
-      if(revertWikiPage(repository.owner, repository.name, from, to, context.loginAccount.get, None)){
-        redirect(s"/${repository.owner}/${repository.name}/wiki/}")
-      } else {
-        flash += "info" -> "This patch was not able to be reversed."
-        redirect(s"/${repository.owner}/${repository.name}/wiki/_compare/${from}...${to}")
-      }
+    val Array(from, to) = params("commitId").split("\\.\\.\\.")
+
+    if(revertWikiPage(repository.owner, repository.name, from, to, context.loginAccount.get, None)){
+      redirect(s"/${repository.owner}/${repository.name}/wiki/}")
+    } else {
+      flash += "info" -> "This patch was not able to be reversed."
+      redirect(s"/${repository.owner}/${repository.name}/wiki/_compare/${from}...${to}")
     }
   })
 
@@ -110,7 +111,7 @@
   post("/:owner/:repository/wiki/_edit", editForm)(collaboratorsOnly { (form, repository) =>
     defining(context.loginAccount.get){ loginAccount =>
       saveWikiPage(repository.owner, repository.name, form.currentPageName, form.pageName,
-        form.content, loginAccount, form.message.getOrElse("")).map { commitId =>
+          form.content, loginAccount, form.message.getOrElse(""), Some(form.id)).map { commitId =>
         updateLastActivityDate(repository.owner, repository.name)
         recordEditWikiPageActivity(repository.owner, repository.name, loginAccount.userName, form.pageName, commitId)
       }
@@ -125,7 +126,7 @@
   post("/:owner/:repository/wiki/_new", newForm)(collaboratorsOnly { (form, repository) =>
     defining(context.loginAccount.get){ loginAccount =>
       saveWikiPage(repository.owner, repository.name, form.currentPageName, form.pageName,
-          form.content, loginAccount, form.message.getOrElse(""))
+          form.content, loginAccount, form.message.getOrElse(""), None)
 
       updateLastActivityDate(repository.owner, repository.name)
       recordCreateWikiPageActivity(repository.owner, repository.name, loginAccount.userName, form.pageName)
@@ -160,9 +161,16 @@
   })
 
   get("/:owner/:repository/wiki/_blob/*")(referrersOnly { repository =>
-    getFileContent(repository.owner, repository.name, multiParams("splat").head).map { content =>
-      contentType = "application/octet-stream"
-      content
+    val path = multiParams("splat").head
+
+    getFileContent(repository.owner, repository.name, path).map { bytes =>
+      val mimeType = FileUtil.getMimeType(path)
+      contentType = if(mimeType == "application/octet-stream" && FileUtil.isText(bytes)){
+        "text/plain"
+      } else {
+        mimeType
+      }
+      bytes
     } getOrElse NotFound
   })
 
@@ -182,5 +190,22 @@
       }
   }
 
+  private def conflictForNew: Constraint = new Constraint(){
+    override def validate(name: String, value: String): Option[String] = {
+      optionIf(targetWikiPage.nonEmpty){
+        Some("Someone has created the wiki since you started. Please reload this page and re-apply your changes.")
+      }
+    }
+  }
+
+  private def conflictForEdit: Constraint = new Constraint(){
+    override def validate(name: String, value: String): Option[String] = {
+      optionIf(targetWikiPage.map(_.id != params("id")).getOrElse(true)){
+        Some("Someone has edited the wiki since you started. Please reload this page and re-apply your changes.")
+      }
+    }
+  }
+
+  private def targetWikiPage = getWikiPage(params("owner"), params("repository"), params("pageName"))
 
 }
\ No newline at end of file
diff --git a/src/main/scala/service/WikiService.scala b/src/main/scala/service/WikiService.scala
index 3d09bc3..8802841 100644
--- a/src/main/scala/service/WikiService.scala
+++ b/src/main/scala/service/WikiService.scala
@@ -19,8 +19,9 @@
    * @param content the page content
    * @param committer the last committer
    * @param time the last modified time
+   * @param id the latest commit id
    */
-  case class WikiPageInfo(name: String, content: String, committer: String, time: Date)
+  case class WikiPageInfo(name: String, content: String, committer: String, time: Date, id: String)
   
   /**
    * The model for wiki page history.
@@ -43,7 +44,7 @@
         if(!dir.exists){
           try {
             JGitUtil.initRepository(dir)
-            saveWikiPage(owner, repository, "Home", "Home", s"Welcome to the ${repository} wiki!!", loginAccount, "Initial Commit")
+            saveWikiPage(owner, repository, "Home", "Home", s"Welcome to the ${repository} wiki!!", loginAccount, "Initial Commit", None)
           } finally {
             // once delete cloned repository because initial cloned repository does not have 'branch.master.merge'
             FileUtils.deleteDirectory(Directory.getWikiWorkDir(owner, repository))
@@ -59,7 +60,7 @@
     using(Git.open(Directory.getWikiRepositoryDir(owner, repository))){ git =>
       optionIf(!JGitUtil.isEmpty(git)){
         JGitUtil.getFileList(git, "master", ".").find(_.name == pageName + ".md").map { file =>
-          WikiPageInfo(file.name, new String(git.getRepository.open(file.id).getBytes, "UTF-8"), file.committer, file.time)
+          WikiPageInfo(file.name, new String(git.getRepository.open(file.id).getBytes, "UTF-8"), file.committer, file.time, file.commitId)
         }
       }
     }
@@ -148,7 +149,7 @@
    * Save the wiki page.
    */
   def saveWikiPage(owner: String, repository: String, currentPageName: String, newPageName: String,
-      content: String, committer: model.Account, message: String): Option[String] = {
+      content: String, committer: model.Account, message: String, currentId: Option[String]): Option[String] = {
 
     LockUtil.lock(s"${owner}/${repository}/wiki"){
       defining(Directory.getWikiWorkDir(owner, repository)){ workDir =>
@@ -158,6 +159,7 @@
         // write as file
         using(Git.open(workDir)){ git =>
           defining(new File(workDir, newPageName + ".md")){ file =>
+            // new page
             val created = !file.exists
 
             // created or updated
diff --git a/src/main/twirl/wiki/edit.scala.html b/src/main/twirl/wiki/edit.scala.html
index a9b8d62..6177974 100644
--- a/src/main/twirl/wiki/edit.scala.html
+++ b/src/main/twirl/wiki/edit.scala.html
@@ -26,6 +26,7 @@
     @helper.html.preview(repository, page.map(_.content).getOrElse(""), true, false, "width: 900px; height: 400px;", "")
     <input type="text" name="message" value="" style="width: 900px;" placeholder="Write a small message here explaining this change. (Optional)"/>
     <input type="hidden" name="currentPageName" value="@pageName"/>
+    <input type="hidden" name="id" value="@page.map(_.id)"/>
     <input type="submit" value="Save" class="btn btn-success">
   </form>
 }