I'm having issues when uploading a new file to an existing item in the media library. About 30% of the time saving the node fails and I get the following error in developer console:

    POST http://xxx/umbraco/backoffice/UmbracoApi/Media/PostSave 500 (Internal Server Error) 
    (anonymous function) 
    @ angular.min.js?cdv=1940264254:106o @ angular.min.js?cdv=1940264254:102g 
    @ angular.min.js?cdv=1940264254:100i @ angular.min.js?cdv=1940264254:79i 
    @ angular.min.js?cdv=1940264254:79(anonymous function) @ angular.min.js?cdv=1940264254:80e.$eval 
    @ angular.min.js?cdv=1940264254:92e.$digest  
    @ angular.min.js?cdv=1940264254:90e.$apply  
    @ angular.min.js?cdv=1940264254:92(anonymous function)  
    @ angular.min.js?cdv=1940264254:156x.event.dispatch  
    @ jquery.min.js?cdv=1940264254:5y.handle  
    @ jquery.min.js?cdv=1940264254:5

angular.min.js?cdv=1940264254:63 TypeError: data.indexOf is not a function
    at umbraco.services.js?cdv=1940264254:6876
    at umbraco.services.js?cdv=1940264254:6957
    at angular.min.js?cdv=1940264254:100
    at o (angular.min.js?cdv=1940264254:80)
    at o (angular.min.js?cdv=1940264254:80)
    at angular.min.js?cdv=1940264254:81
    at Object.e.$eval (angular.min.js?cdv=1940264254:92)
    at Object.e.$digest (angular.min.js?cdv=1940264254:90)
    at Object.e.$apply (angular.min.js?cdv=1940264254:92)
    at j (angular.min.js?cdv=1940264254:101)

The response contains the following stacktrace:

{
"Message": "An error has occurred.",
"ExceptionMessage": "The process cannot access the file 'D:\\home\\site\\wwwroot\\App_Data\\TEMP\\FileUploads\\BodyPart_4da9803d-6452-4c5a-b491-4df24e9912fa' because it is being used by another process.",
"ExceptionType": "System.IO.IOException",
"StackTrace": "   
    at System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath) 
    at System.IO.File.InternalDelete(String path, Boolean checkHost) 
    at Umbraco.Web.WebApi.Filters.FileUploadCleanupFilterAttribute.OnActionExecuted(HttpActionExecutedContext actionExecutedContext) 
    at System.Web.Http.Filters.ActionFilterAttribute.CallOnActionExecuted(HttpActionContext actionContext, HttpResponseMessage response, Exception exception) 
    at System.Web.Http.Filters.ActionFilterAttribute.<>c__DisplayClass2.<System.Web.Http.Filters.IActionFilter.ExecuteActionFilterAsync>b__1(CatchInfo`1 info) 
    at System.Threading.Tasks.TaskHelpersExtensions.<>c__DisplayClass4`1.<Catch>b__3() 
    at System.Threading.Tasks.TaskHelpersExtensions.CatchImpl[TResult](Task task, Func`1 continuation, CancellationToken cancellationToken)"
}

We're running Umbraco 7.2.6 on Azure Webapp Basic.