How to retrieve Sitecore media item from the URL we have?
The URL is dynamic URL e.g. /~/media/14BDED00E4D64DFD8F74019AED4D74EB.ashx
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This is generated when you add item in rich text field.
When adding an item in the Rich Text field, you can use the FieldRenderer to render out the output - Sitecore will then create the correct URL automatically. That way you won't even have to worry about the URL.
The FieldRenderer control can be used like so:
<sc:FieldRenderer ID="renderer" runat="server" FieldName="fieldname" />
Or if you're using XSLT:
<sc:text field="fieldname" />
In codebehind you could do something like
FieldRenderer.Render(Sitecore.Context.Item, fieldname);