Fix thumbnails computation when importing a large PDF with S3 BlobProvider

Issue Description:
Thumbnail is not getting created when a greater than 5 MB PDF file is added to a folder using the "Create" button with a S3 BlobProvider

Build a configuration with a S3 instance (e.g. direct download, direct upload) with hotfix up to 10.10-HF37

Steps through which this issue is reproducible in this environment:

• Navigate to a folder in Nuxeo web UI
• Click on the "+" icon to Create a new document
• In the popup window that comes up, click on the "Import" tab
• Drag and drop a pdf file (which is greater than 5 MB for the purpose of this test) into the "Import tab"
• After the pdf file is uploaded, Click on the "Add Properties" button
• Select the "Asset Type" as "File"
• Click on "Create" button to create the file
• After the file is created, click on the breadcrumb and navigate back into the folder
• Within the folder listing view, the thumbnail of the PDF content is not showing for the uploaded PDF file (instead it shows the default "File" document type image for the thumbnail)

One observation is in the case where thumbnail gets generated correctly, the mimetype of the main document shows as "application/pdf". Whereas in the case where the thumbnail doesn't get generated, the mimetype of the main document shows as "application/pdf; charset=UTF-8"