Delete file in Get method web api

Question

I have an issue in my Web Api when I try to delete some files from the server, i get this error

"The process cannot access the file because it is being used by another process"

My delete method is called in this get method :

public IQueryable<FriendChallenge> GetJudgedChallenge(string id)
        {
            var friendChallenges = challengeFriendRepo.GetFriendJudgeChallenge(id);
            var query = friendChallenges;

            ChangeFriendChallenge(friendChallenges, "");


            query.ForEach(fc => fc.Challenge = challengeRepo.Get(fc.ChallengeId));
            query.ForEach(fc => fc.Friend = friendRepo.Get(fc.FriendId));

            query.ForEach(fc => fc.Challenge.User.UserName = AESCrypt.DecryptRijndael(fc.Challenge.User.UserName, fc.Challenge.User.SecurityStamp));

            deleteFile(friendChallenges);

            return query.AsQueryable();
        }

To be sure that i can delete the file, I have to call delete method from this get method.

So my delete method loop in my list and i get the filename to create the path :

        string localFilePath = AppDomain.CurrentDomain.BaseDirectory + @"\ChallengeContent\";
        localFilePath += imagePath;

        if (File.Exists(localFilePath))
        {
           // Content.Dispose();
            File.Delete(localFilePath);
        }

The file exist, but when the File.Delete is call i get the error...

All my Read method are done in a using, the write is a multipart method.

EDIT :

My write method :

 public async Task<HttpResponseMessage> UploadFile()
        {

            // Check whether the POST operation is MultiPart?
            if (!Request.Content.IsMimeMultipartContent())
            {
                throw new HttpResponseException(HttpStatusCode.UnsupportedMediaType);
            }

            string appPath = AppDomain.CurrentDomain.BaseDirectory + @"\ChallengeContent\";
            string fileName = String.Empty;
            if (Directory.Exists(appPath) == false)
            {
                Directory.CreateDirectory(appPath);
            }

            CustomMultipartFormDataStreamProvider provider = new CustomMultipartFormDataStreamProvider(appPath);
            List<string> files = new List<string>();


            try
            {
                // Read all contents of multipart message into and save files in the server
                await Request.Content.ReadAsMultipartAsync(provider);

                foreach (MultipartFileData file in provider.FileData)
                {
                    files.Add(Path.GetFileName(file.LocalFileName));
                }

                // Send OK Response along with saved file names to the client.
                return Request.CreateResponse(HttpStatusCode.OK, files);
            }
            catch (System.Exception e)
            {
                return Request.CreateErrorResponse(HttpStatusCode.InternalServerError, e);
            }


        }

EDIT 2 :

Sorry I did not think to add it with in the previous edit

  public class CustomMultipartFormDataStreamProvider : MultipartFormDataStreamProvider
    {
        /// <summary>
        /// multipart provider
        /// </summary>
        /// <param name="path">path</param>
        public CustomMultipartFormDataStreamProvider(string path) : base(path) { }

        /// <summary>
        /// Get local file name multipart
        /// </summary>
        /// <param name="headers">http header</param>
        /// <returns>file name</returns>
        public override string GetLocalFileName(HttpContentHeaders headers)
        {
            return headers.ContentDisposition.FileName.Replace("\"", string.Empty);
        }
    }